I've found that there are a lot of different schools of thought on this one. Some coaches suggest that runners don't need legs and shouldn't do lower body weights at all. They think doing lower body weights will up the potential for injury and over-stress the legs.
Another coach that I read articles from suggests that lower body weights are absolutely necessary to train the muscles in your legs that you don't use as much when you run. Since those strong muscles in your quads, etc, might pull more strongly on your knee (for example) in one way than they would if your muscles were worked more evenly, you might be more prone to injury and pain.
I take the middle road - I am in the off season right now, and so I'm doing the full STS system including legs, even though I'm still running 3-4 miles twice a week on weekdays and up to 10-13 miles on Saturday (although I think I average about 7 miles on Saturdays). When I get more fully into the season, I will be done with STS and I will still do lower body legs, but a less intense version - really just some squats, lunges and things to keep myself in check. Once I get into the REALLY long runs - 20+ miles on Saturdays, I will probably go even lighter, still doing lower body, but using body weight only.
I think it depends a lot on WHEN you do the weight training - honestly, the ideal thing to do is run and do lower body weights on the same day so you can give your legs a full day off, but that doesn't fit into my schedule 90% of the time. I know lots of runners that do weight training on their lower body and never get injured and are in fact better runners because of it, and I know some that never weight train and still do great. It's all balance!
Regardless, I don't think your legs will get too muscular, they will be more shapely. Rather than looks, I would be more concerned about injury potential.