Why don’t most runners run the full marathon distance (26.2 miles or 42.2 kilometers) in training while training for a marathon?
When it comes to training, time is what matters, not the distance. Time represents the amount of stress. The reason why it’s not a good idea to run the full marathon distance in training is because of how long it takes most runners to do that and the consequent muscle fiber damage incurred and subsequent recovery time needed.
By contrast, elite runners regularly run more than 26.2 miles (42.2 km) in training because they run the distance so fast that it doesn’t take much time. In my published study on the 2004 U.S. Olympic Marathon trials qualifiers, I found that the average length of the long run for men was 25 miles (40.2 km) (range = 20-52 miles) and 23.5 miles (37.8 km) (range = 18-30 miles) for women. For the year leading up to the Olympic Trials, the men ran longer than 20 miles (32 km) an average of 17.7 times (range = 1-60 times), while the women did so an average of 10.4 times (range = 0-50 times). The large ranges shows there were big differences between runners in the length of the long run and in how many times the runners ran longer than 20 miles.
If a run takes you more than about 3.5 to 4 hours to complete, it’s going to take a lot of time to recover from that, and will thus get in the way of the other training you need to do during the rest of the week.
While most recommendations are to cap your long run at 20-22 miles (32-35 km), there’s no law that says you can’t run longer. It’s all about the time, and finding out where the point of diminishing returns are for you. For most runners, several to many 20- to 21-mile runs is likely better preparation for a marathon than one 25-mile run.
So give yourself plenty of time to prepare, and work up to handling several very long runs.
(To read the study on the U.S. Olympic Marathon Trials qualifiers, go to run-fit.com/research)