The Bolder Boulder is the most famous (and enjoyable) race in Denver.
It's run every Memorial Day and is for both the very competitive and the relaxed runner/walker.
People run in clown suits, everyone does the "Chicken Dance," and thousands of singers, belly dancers, bands, and citizens with hoses line up along the residential streets of downtown Boulder. People can walk or run, or stop and belly dance. For the non-expert, it is a silly and child-like good time. At the end of the race, free food and drink, as well as beer samples, etc. are set up next to Folsom Field, home to the CU Buffs.
It is important to register in advance. You can do so online at Bolder Boulder or at various locations around Denver.