After Bernard Lagat competes Saturday in the Fifth Avenue Mile, he plans to do something that would feel completely foreign to many elite runners: He will toss his sneakers in a closet and pig out for the next five weeks. No running. No sit-ups. No heavy lifting, except for a fork.

For a handful of top-notch athletes, including Lagat and fellow U.S. Olympians Matthew Centrowitz and Leo Manzano, Saturday's race will mark the end of a taxing year of competition. There were long training runs last fall, indoor races last winter...