Diane Reynolds had been racing for a few months when she won her first amateur cycling event, the Farm to Fork Fondo near upstate New York’s Finger Lakes in August. She left more than 500 riders in the dust, including all the men.

The win earned the 49-year-old novice a jersey decorated with polka-dot chickens, but it didn’t come cheap: She paid about $1,000 for former pro cyclist Hunter Allen to ride all 84 miles with her as a private coach.

Mr. Allen, 50, gave her real-time pointers on pacing, technical skills and race strategy. He also ran interference for her. “Early on, there were about 10 guys riding hard taking turns up front—I was one of them—and I knew we were going to break away from the peloton,” or main group of riders, he says. “I made sure Diane stayed with us, sheltered in the middle and conserved her energy as we widened the gap.”

Dr. Reynolds, an anesthesiologist in Knoxville, Tenn., ended up setting a record as the only female overall winner in 27 Farm to Fork Fondo events, according to organizers. “It wasn’t like I qualified for the Olympics, but I hit my personal goal and that was a great feeling,” she says.

While coaches have long attended amateur races to support their clients from the sidelines, more are joining them on the starting line as pacers-for-hire. Instead of competing outright, they agree to race alongside their client as an ally. That means registering, putting on a bib and finishing with an official time—often torpedoing their own race to help a client achieve a goal.