Start early
Schedule your race as early in the morning as possible - especially in warm weather months. If you offer two events, start the longer one first, so runners who spend more time on the course can do so before it gets too hot.
Runners are generally advised to run early in the morning when exercising or training on their own. Races should follow that advice, too.
Note: An ideal starting time for short distance races (5k and 10k) held in warm weather months is 8:00 a.m. (earlier is OK, too). Longer distance races should start earlier than that, to reduce exposure to sun and heat.
The Bank of America Chicago Marathon, in October, starts at 7:30 a.m. The Honolulu Marathon begins at 5:00 a.m., allowing the majority of runners to finish before the heat becomes too severe.