i am not a runner, but can honestly say that running alone will not bulk up your thighs. it is the same with cycling as people think that will really bulk your legs up. these athletes do alot of weight training on their legs, hence bulk.
have a look at marathon runners. they are the most spindly people i have ever seen and they run for hours on end.
i think if anything you will find your calves become more defined, but you shouldn't gain any mass on your legs really.
just my opinon