Exactly How Long (and When) Your Longest Marathon Training Run Should Be

Take a peek at any marathon training program, and you’ll inevitably see a few long runs on the schedule. These workouts are essential to getting to the starting line of your race feeling strong and confident. (After all, you wouldn’t want to go from 0 to 26.2!) However, you might be surprised to hear that the longest run before a marathon typically isn’t the full race distance. 

So how many miles should your longest run before a marathon be? And when, exactly, should you get it done? We tapped three running experts to discuss why long runs are important, how to tackle your longest marathon training run, what to know about fueling it, and more. You’ll be ready to hit the ground running (literally) in no time. 

Why Long Runs Are an Important Part of Marathon Training

Long runs are a key part of marathon training because they help build your mental toughness and cardiovascular (i.e., aerobic) fitness, according to Victoria Sekely, a doctor of physical therapy and running coach. “These runs help you build the endurance and stamina that you’ll need to get to the start line of your marathon prepared,” adds Peloton instructor Susie Chan.

Specifically, long runs boost your running economy (how much oxygen you need to run at a certain pace), teach your body how to efficiently use glucose (a main source of energy), and increase the amount of blood your heart pumps with each beat, according to Kimberly Melvan, a doctor of physical therapy and running coach. All these changes help equip your body to run 26.2 miles. “You get valuable endurance adaptations by spending time on your feet,” Melvan explains. 

Long runs also prepare your musculoskeletal system for the physical impact of running the full marathon distance, Sekely explains. You can’t just jump from, say, five miles to 15 miles and expect your body to tolerate it well. “Just like when you build muscle, you need to take your time and make sure you don’t do too much too soon as you run the risk of injury,” she says. 

The key is to build up to the longer distances slowly and steadily, Susie says. “You want to make sure you don’t make any huge jumps in mileage, as this will help you keep injury-free,” she says. Research backs this up: Runners who increase mileage too fast have a higher risk of injury compared to runners who gradually up their mileage, according to a 2023 study published in the British Journal of Sports Medicine. 

How Long Your Longest Run Before a Marathon Should Be

“Generally, anything from 18 miles to 22 is the sweet spot of your longest run,” Susie says. Where you fall within that range will depend on your level of experience and the specific training plan you’re following. For instance, Peloton’s Road to Your 26.2 marathon training program includes a 20-mile long run, whereas the advanced training plan from the Boston Athletic Association (which organizes the Boston Marathon) maxes out at 22 miles.

Melvan believes that, psychologically, 20 miles is a good goal for peak mileage because “knowing you can hit 20 miles lets you know you’re close to the full distance without increasing your injury risk by running more,” she says.

Research supports the fact that your longest marathon training run should be around 20 miles. A 2020 study published in the Scandinavian Journal of Medicine & Science in Sports found that marathon runners who did a longest run of more than 25 km (about 15.5 miles) had faster marathon finish times than those who ran less than that during their longest run. Additionally, those who did a longest run of more than 35 km (about 21.7 miles) saw no additional benefits in their race times. 

The other thing to keep in mind when training for a marathon—and programming your longest run—is your total weekly mileage, Sekely says. “It’s the cumulative volume of your runs combined throughout the week that prepares you most,” she says. “It’s how you know you don’t have to run the full marathon distance in order to be prepared for the race.” She typically recommends your long run be no greater than 50 percent of your total volume for that week, which means if you have a 20-mile long run on the weekend, your total weekly mileage should be at least 40 miles that week.

When to Do Your Longest Run Before a Marathon

Again, this will depend on your race training plan; however, Susie, Melvan, and Sekely all recommend that you do your longest run about three to four weeks out from your marathon. This way, you’ll give your body enough time to recover and taper down your training before the big day.

Tapering is a term used to describe dialing back your volume and intensity in the weeks leading up to your race so you can recover from all the training you’ve done and be fresh and ready to go on race day. Don’t worry—you won’t lose all the fitness you’ve built up. A 2021 study in Frontiers in Sports and Active Living found that runners who tapered about three weeks before their marathon finished the race an average of five minutes faster than runners who didn’t. 

Your longest run before your race is also a great time to test out any gear or nutrition products, so doing it a few weeks ahead of your marathon gives you enough time to tweak things if necessary, Susie adds. 

At What Pace Should Your Longest Pre-Marathon Run Be? 

For the bulk of your marathon training, your long runs should be done at a “conversational” or “easy run” pace, according to Melvan. But the way you pace your longest pre-marathon run will also depend on your experience and goals.

For example, if you’re a more advanced runner or targeting a certain finish time, you can try to complete your longest pre-marathon run at your ideal race pace so you’ll know what it feels like, Susie says. “Or you could try running some ‘over and unders’ where you divide the running between slower than marathon pace and slightly faster,” she adds. “The average pace will end up being your marathon pace.” You could also start your last few long runs at an easier pace and then add in goal- or marathon-pace miles towards the tail end of each run, Melvan says.

A general guide is to keep your running heart rate to around 70 to 80 percent of your max heart rate for your long runs—or low enough that you can hold a conversation while running, Melvan says. This slower running pace helps you build endurance without overtaxing your body. 

Whatever your strategy, keep in mind that it’s important to listen to your body and do what feels best for you in the moment. 

Why You Typically Don’t Want to Run the Full Marathon Distance Before the Race

For most runners, there’s no need to run the full marathon distance during your training, Susie says. Running the full 26.2 before race day can lead to excessive fatigue and increased injury risk and may leave you physically and mentally drained, Melvan explains. If you were to run a full marathon during training, your body would need a significant amount of time to recover, Susie adds—and that could ultimately disrupt your training plan. “You might be tempted to see what it’ll feel like, but save that for the big day,” she says.

If you complete a longest run of 18 to 22 miles, you’ll have built up enough physical and mental endurance to make it through the full marathon distance; race-day magic will carry you the rest of the way. “I like to say that if you’ve trained properly, the adrenaline and excitement can help you the most in those last few miles,” Melvan says. Susie agrees: “On the day itself, you’ll have the extra drive of it being the actual race, and that can be so motivating!” 

How to Fuel for Your Pre-Marathon Long Run

Fueling for your long run is essential to maintaining energy levels, but you need to eat intentionally. These strategies can keep you from crashing and help prevent runner’s stomach, which refers to a whole host of GI issues like diarrhea, cramping, bloating, side stitches, and heartburn.

Start by eating a balanced breakfast two to three hours before the run, including simple carbs and a small amount of protein, Melvan says. (This could look like oatmeal, eggs, or toast topped with peanut butter.) Simple carbohydrates, sometimes called “fast-acting carbs,” are easily digestible and can be used for fuel immediately, whereas protein keeps your blood sugar stable and prevents muscle breakdown. You’ll also want to drink 17 to 20 ounces of water two to three hours before your run and 7 to 10 ounces of water 15 to 20 minutes before your run, Melvan says.

During the run itself, Sekely suggests taking in 60 to 90 grams of carbs per hour to keep your energy levels high. Gels or chews are great options for on-the-go fuel that you can store in your running belt or pocket. 

Make sure to hydrate well during and after the run, too, Melvan says. Consider taking two to three gulps of water every 15 to 20 minutes during your run and incorporating electrolyte-rich drinks to replenish sodium, potassium, and magnesium. Electrolytes are essential minerals that are vital to many bodily functions, per the National Library of Medicine, but you lose them in your sweat. Sweating for an extended period of time—including when you’re running long distances—can leave you deficient.

“Use your long runs during training to experiment with fuel and hydration so that when race day comes, you have a solid plan and aren’t trying anything new,” Melvan says. “There are numerous factors, internal and external, that can affect how much you’ll need to take in, so adjust for that. Elements such as weather, time of day, your overall energy level, type of fuel, and distance of your run can all play a role.”

Tips for Recovering After Your Longest Marathon Training Run

Post-run recovery is crucial to prepare for the next phase of your race prep. To kickstart recovery after your longest run, make sure to drink a lot of water and fuel properly in the days following, Melvan says. 

That includes consuming a post-workout snack or meal within 30 minutes after your run that includes both carbohydrates and protein, Sekely says. For instance, you could eat a burrito bowl with rice, veggies with a protein of your choice (such as chicken or tofu), or a bowl of oatmeal with fruit and nut butter.

You’ll also want to incorporate some physical activities that can help speed up muscle repair and reduce soreness. “Any type of gentle movement helps with blood flow and recovery,” Melvan says. Yoga or mobility work might feel good to you, or you can get your blood pumping with some light cardio. A small 2025 study in Scientific Reports, for example, found that low-intensity aerobic exercises—like light cycling or jogging—can help reduce muscle soreness and facilitate recovery. “One of the best things to help the legs is gentle walking the day after,” Susie adds.

Foam rolling and massage can also help with tired legs. Foam rolling after an intense bout of exercise, like sprinting, can help reduce muscle pain associated with delayed onset muscle soreness (DOMS), according to a 2019 review published in Frontiers in Physiology. What’s more, a 20- to 30-minute massage performed immediately following or up to two hours after exercise has been shown to effectively reduce DOMS for 24 hours—and even up to 96 hours—post-exercise, per a 2018 review in Frontiers in Physiology.

Last but not least, ensure you're getting enough sleep, as it allows your muscles to repair and rebuild, according to both Sekely and Melvan. 

The Takeaway

Long runs are one of the most important aspects of training for a marathon, as doing them helps you prepare for the distance not only physically but also mentally. They help you build the endurance, stamina, and mental toughness needed for a strong race finish and also act as a trial run for marathon day. You can test out gear, hydration, and fueling strategies to learn what works for you—and what doesn’t. 

In general, you should do your longest marathon training run between 18 and 22 miles about three weeks before your race, as running the full 26.2 shortly before the event can leave you fatigued and even injured. You want to be prepared—but with enough energy still in the tank to make it to the big day.