The 20 Mile Rule

Build up to and run at least one 20 miler before a marathon. Long runs simulate the marathon, which requires lots of time on your feet, says Gina Simmering Lanterman, director and marathon coach of the Denver Fit training program. And knowing that you can run 20 miles helps you wrap your head around running 26.2. The Exception Some coaches believe experienced marathoners can get by with a longest run of 16 to 18 miles, while other coaches suggest runs up to 24 miles.