Well if you run every day, training your legs might go a little into the realm of overtraining as they already get a fair share of work from your running but that depends on your ability to recover and how intense the running is.
Maybe you can try doing something like squats but except with 1 or 2 sets rather than the usual 3 or 4 sets and only once a week.
Legs are essential. Squatting releases more testosterone than any other lift. As such, squatting will cause all of your other lifts to increase. I always do leg day at the beginning of the week because it helps increase my gains throughout.