Why people run marathons—and why any type of cardio is good for you

In 2025, Denver’s Colfax Marathon saw 25,000 participants, marking the largest crowd to date. So, why are more people signing up for these increasingly popular events? Kevin Masters, Ph.D., professor of clinical health psychology at CU Denver, first set out to answer that question with a series of research studies in the 1990s and early 2000s.