The most important areas to target are your knee extensors, hip adductors, hip abductors, hip flexors, hip extensors, gluteals, soleus, and gastrocnemius.
Three sets of 10 reps with light weights is the prescription here. The emphasis is on isolating the targeted muscle so you don’t want to heave around as much weight as possible, because that would encourage supporting muscles to kick in and do some of the work. Remember, the goal is to strengthen specific, often minor muscles, not to build your overall strength and general buffitude.
For example, let’s say I’m targeting my knee extensors (thigh muscles above the knee toward the midline of the body). I do these seated, with an 8-pound ankle weight, straightening my lower leg by contracting my knee extensors. The sensation in the muscles I’m targeting is of a little extra work than if I were extending my leg without the weights. Of course I could do the same exercise with more than three times the weight, but at some point, to raise my leg I’d start recruiting hip muscles and other quad muscles than the ones I’m targeting.
Aim for two to three leg strengthening sessions per week, ideally on your easiest running days.