The Boulder Marathon course is primarily on hard-packed dirt roads (85%). The section of the road on Nelson road is paved. The start/finish is also on a paved road (but that is only for about a quarter of a mile - until the runners and walkers exit the Boulder Reservoir). The rest of the marathon course is on dirt roads.
See what splits you need to run for hundreds of different marathons - adjusted for the specific terrain of each marathon, your goal time and pacing strategies.