There aren't too many races in Dallas proper this weekend, but I recommend the Race for the Cure in Fort Worth, if you like big, popular races. The Komen events are always fun and full of energy. With lots of pink, of course.
Or if you're in the mood for a little road trip, Decatur's a nice spot. You can really feel at one with nature while you're there, especially with these folks at their 5K Sun Run. It's part of a series of 5Ks, if that's your thing.