We like to spend what free time we have running on the trails, which works the "front to back" muscles, but neglects the "side to side" muscles like the hip abductors. When the hip abductors are weak or fatigued, it becomes difficult to stabilize the pelvis in the frontal plane, especially since we are only in contact with one foot on the ground at any given time while running. Because everything is connected, weak outer hip muscles cause the entire leg to overpronate: opposite hip "drops", knees "cave in" (potential cause of anterior knee pain aka patellofemoral joint pain, patellar tendonitis, IT band syndrome), foot overpronates (potential cause of achilles tendonitis, plantar fasciitis, peroneal tendonitis, posterior tibialis tendonitis), etc. It also means that something else has to try to compensate for the weakness (TFL, quadratus lumborum, piriformis). This is a bigger problem for women because we have a wider pelvis, and therefore a longer moment arm.
Weak gluteus medius
The good news is, it's easy to strengthen these muscles, even without any special equipment. There are several ways to accomplish this, the easiest being side lying straight leg raises. Be sure that you maintain proper alignment - an easy way to do that is to lay on your side with your back against a wall. As you raise your top leg, make sure that your foot remains in contact with the wall (kind of like doing snow angels with one leg).
A more advanced version will also strengthen the core, especially the quadratus lumborum in the low back: hip abduction from a T stand.
Other options include lateral tube walking (put a loop of elastic tubing around your ankles and walk sideways) or standing hip abduction using a low cable (try not to hold on to anything with your hands).
These are just a few options for strengthening the hip abductors. Remember, their job is primarily stabilization, so go for endurance rather than maximal strength. Try just one, or do them all if you wish.
As Nike says, Just Do It.