Back-to-Back Long Runs (for Ultras)

What they are for

The job of a back-to-back is rehearsal: pace, food and terrain on tired legs, in the race's shoes on the race's kind of ground. The durability itself comes from regular long runs inside the three-hour limit and the easy weekly running around them.

A long run does its work between roughly 90 minutes and three hours: most of a run's aerobic benefit comes in about its first two hours, and a run past about three hours is hard to recover from, costing you training in the days after it. Longer long runs mostly make you slower (see the guide on long runs). In 26 well-trained men matched on 10 km time, those who regularly ran 90 minutes or longer lost less running economy over a 90-minute run than those whose runs were all under 70 minutes (3.1% against 6.0% worse)¹. The runners who held up best also trained the most each week, so the study cannot separate the long runs from the volume around them.

How long each day can be

A three-hour run followed by a run of two to two and a half hours is a full back-to-back, and there is no reason to push day one past three hours.

Size each day to the running you do now, not to the longest run you once did. In 5,205 runners followed for 18 months, injuries were more common when a session was more than 10% longer than the longest run of the previous 30 days: about 1.6 times as often for a session 10% to 30% longer, 1.5 times for 30% to 100% longer and 2.3 times for more than double². So build day one in small steps.

Stay at two days. A third long day in a row adds load, not practice.

Between the two days

Eat carbohydrate and a protein source soon after day one, and keep eating through the evening. A 2021 review advises at least 1.2 grams of carbohydrate per kilogram of body mass per hour for the first few hours after a long session to restore muscle glycogen for the next one³. For a 70 kg runner, that is about 85 grams an hour. A 2004 review of the training diet advises adding a good source of protein to recovery meals and snacks⁴. Total protein for the day matters most: the International Society of Sports Nutrition's 2017 position stand puts 1.4 to 2.0 grams per kilogram of body mass a day as enough for most exercising people, spread across the day, about every 3 to 4 hours⁵.

Use day two to practice race fueling. See the guides on training your gut and carbohydrate during exercise.

Related guides

For the long run itself, see long runs. For ultra training as a whole, see ultra training.