Dick Costolo, the affable former C.E.O. of Twitter, was well into his 84th burpee when the beads of sweat that clung to his forehead finally succumbed to gravity and fell to the floor, cannonballing into a small puddle of his perspiration that had formed beneath him. Costolo hardly noticed as he once again flung his feet backward and dropped his chest to the floor and let out a pained exhale. If he did, he was too exhausted to notice. We were standing in the middle of a San Francisco CrossFit gym, and Costolo was grunting—sometimes wiping his brow—as he worked out with two people (and one reporter) about half his age. Costolo blurted out a prostrated “ugh” as he launched into burpee No. 85, then another for the 86th. Finally, after he had completed his 100th repetition, he hunched over to rest his chalky palms on his kneecaps, straining to catch his breath as his cobalt-blue Nike moisture-wicking T-shirt struggled to do its job.

This is what constitutes as a daily workout for him these days: one crafted by his new co-workers earlier that morning, and one in which—this being San Francisco and a CrossFit gym—they had diagramed on the white board that lined the gym’s walls. To most people in Silicon Valley, Costolo is best known as the C.E.O. who took Twitter from white-hot phenomenon to a public company with some $2 billion in annual revenue. But to the other people hoisting barbells over their heads in the gym, he was another guy finishing off what in the lingua franca of CrossFit is known as a “chipper”—which roughly translates to a descending ladder of repetitions of a prescribed series of exercises. This afternoon, the chipper called for 50 burpees, 40 push-ups, 30 calories churned out on a rowing machine, 20 toes-to-bar, 10 pull-ups, and then a repetition of all of the above.

The choreographed routine was timed by a red digital clock that was intended to motivate Costolo and his companions, inject a little healthy competition between them, and provide a number to beat next time. But the clock also had the inverse effect of humbling Costolo, the elder figure in the group at 53, as he was beaten to the finish by Bryan Oki, a CrossFit coach whom Costolo hired to teach classes at Twitter years ago, and Katie Everett, a former Twitter software engineer who rowed crew and swam at M.I.T. Both Oki and Everett finished the routine in a little more than 20 minutes. As Costolo pushed through his final burpees, each second seemed to last an eternity. Nevertheless, he finished the set diligently, without so much as a sideways glance or cheating on the final few reps.

Costolo’s next act has been a subject of Valley speculation since his departure from Twitter more than one year ago, in part because his variegated past makes his future anyone’s guess. He was a former improv comic turned serial entrepreneur turned C.O.O. turned C.E.O. But he also happened to be a gym rat. And so earlier this year, Costolo decided to merge those identities. In January, he announced on Twitter, naturally, that he would be creating a fitness software start-up, Chorus, which is due to launch in December. (He is also joining the venture-capital firm Index Ventures as a partner.) Chorus, which now has an eight-person team and an initial investment of $8 million, aims to take the emerging phenomenon of group fitness, such as SoulCycle or CrossFit or countless other boutique models, and bring it to scale, like a tech company would. Its software platform, which consumers will be able to access with a subscription fee, will link up athletes who share common fitness goals, like training for a marathon, increasing their functional strength, or more ephemeral health resolutions, like sleeping better. The logic that undergirds Chorus, much like Weight Watchers or A.A., is that people who are serious about a goal are better served through positive peer pressure. Costolo often cites himself as an example. He might not have completed that 100th burpee, after all, had it not been for the presence of Oki and Everett.

“When you’ve got 38 burpees left and you’ve already done 62, you stop caring. That’s why I love these kinds of workouts so much,” he said, still catching his breath once he finished.