John Oakley didn’t set out to run three marathons in six weeks this fall. But when he mapped out his training schedule, it just made too much sense not to.

Mr. Oakley, a 54-year-old police sergeant from Bayport, N.Y., initially signed up for last Sunday’s Marine Corps Marathon in Washington. Then he received a free entry into the Hamptons Marathon, held on Sept. 26, so he decided to run it as a 26.2-mile tuneup for the Marine Corps race. During the summer friends encouraged him to run the Suffolk County Marathon on Sept. 13....