I've only done one "fun run." It was not, in fact, "fun," and I may have almost punched a child dressed as a strawberry who abruptly ran out in front of me. THAT BEING SAID, Portland Parks and Rec just announced a "$5 5k" series, which is cheap enough that I'd give the whole thing (strawberry included!) another shot—it's a series of races taking place at different Portland parks, and registration for each is only $5.
For details and registration info, go here.
(WAIT, what if we got a Blogtown team together for one of the races? Eh?