Jimbo brings up another point; it has alot to do with your frame. When i was training for a half marathon as an overweight 210 pound 5'11' guy, my body eventually broke down and i lost a month or so of my training time.
And it's cheap, but make sure you get shoes that fit properly....thats when a running shoe store is worth the extra cost.
riding fixie is good for your running imo. you use a full range of motion, so you don't just have big ol quads and feeble hammies. Calves the size of melons. etc.