I’m going to support as well as argue against some of what COM said.

Yes, running uses more than just your calves and thighs, so if you’re going to train muscle groups, you need to train globally. Calves, quads, hamstrings, hip flexors, glutes, core, even some arms.

About what COM said about weight training being counter-productive for long-distance runners, I dispute that. You do need quite a bit of muscular strength to do long-distance runs, not just for short-distance sprints, and running alone will often not give you that strength you need. So yes, I’m a big supporter of runners doing regular weight training, not only because it will give you the base of strength your muscles need to hold up during a long duration of activity, but also because weight training will also often correct improper biomechanics that will transfer over to your running form. (As an anecdote, as a long-distance runner myself, my performance shot WAY up when I first added resistance training to my regular routine.)

That being said, no way should you only be doing weight training. You still need to be practicing the running distances you intend to compete at. Also, even while weight training, don’t isolate. It will not help your running at all to train one muscle at a time, as you’re not using just one of them at a time while you run. (Especially calves, I feel the need to say. Never, EVER isolate calves. It’s a quick road toward shin-splints.)