If you’re trying to avoid or come back from injury
Many runners wind up with overuse injuries such as IT band syndrome, shin splits and tendonitis. Splitting apart a run may be a safer way to build up endurance and maximize recovery. “A 30-minute run might be a bit too aggressive for an athlete returning from injury,” Dollar says. “However, the same athlete may easily tolerate 15 minutes in the morning and 15 minutes in the evening—yielding them 30 minutes of volume.” The recovery time between the runs will help inflammation subside, allowing the athlete to bounce back easier. Another option may be to do the continuation run—start your long run in the evening and finish it the morning after.