Boulder Half Marathon 2025 Registration. On sunday, september 29th, 2025 for the fourth annual boulderthon marathon in stunning boulder, colorado. Sign up for boulderthon 2025:
You’ll start and finish in the iconic running mecca of downtown boulder, with stunning views of the flatirons throughout the course. Attracting athletes from all over the west due to its phenomenal scenery, glamorous finisher medals, perfect running temperatures with.