Boulderthon Marathon

Growth has been the story here. The race has roughly doubled its field year over year since it started, sold out every edition, and now anchors the largest September running weekend in the country. That has consequences for planning: online entry is the only path, there is no race-morning signup, and waiting on a late decision usually means missing it. Deferrals and transfers exist for a fee, refunds do not. The full marathon is open to runners 13 and up.

Runners leave downtown on Pearl, head out North 51st Street past Boulder Reservoir, and spend the middle third on the county back roads under the Flatirons that local pros use for training. The route then turns back toward town for a finish on the pedestrian Pearl Street Mall, which is a genuinely unusual last quarter mile for a road marathon. A course rebuild cut marathon climbing to about 603 feet from roughly 983, so what is left reads as rolling rather than hilly, and the profile is no longer the hard part. Surfaces are 98 percent paved road and paved multi-use path, with one short packed-gravel stretch.

Late September mornings in Boulder often start near 40 degrees and warm fast under thin high-altitude sun, which is why the organizers push gear check hard. Temperature is not usually the problem. Air is. Start and finish sit around 5,335 feet and the course never drops below that, so a familiar pace costs more oxygen than it does at home and visiting flatlanders tend to pay for it in the last 10K whether or not the day stays cool.