At 21 years old, Thirumurthy ran his first marathon just a few months after graduating from Oberlin. Working in New York at the time, Thirumurthy's friend got him a free last-minute entry to the 1998 NYC Marathon. In a field of over 30,000 racers, the Thirumurthy placed 340th overall, finishing in 2:48:38.
Since then, Thirumurthy has totalled 30 marathons, including a personal-best 2:33 in the 2009 Boston Marathon, placing 72nd overall. Thirteen years later, at the 2022 Boston Marathon, Thirumurthy finished in 2:35, placing third in the M45-49 division. Thirumurthy said that was one of his most memorable marathon performances as he received his award at the same ceremony as the professional runners from the elite field.