I'm currently taking Karate due to a required PE credit. It's pretty slow paced, which is probably good, but it's pretty fun. I've thought about getting into it for real, but I don't think distance running and marshal arts go together very well, mainly due to the flexibility issue. I think if I were to give it a serious go I'd have to stretch like a mad man for at least a year just to properly execute many of the kicks.
Anyone do both?