Good question what builds that "soccer muscle". Depends on how strong it is to start with. Try standing on one leg and gently beginning to squat down. Feel the muscle on the inside/front of the standing leg down near the knee, and see if you can feel it working as you begin your one-legged squat. If you feel a substantial muscle, that's great. Then you can further strengthen it by doing squats, playing soccer, and running, for example. If, like many girls, this muscle is so weak that you can't even feel it (particularly on your back fencing leg), or if it's a lot weaker than your quadriceps, then you'll want to be very gentle as you begin to strengthen it, so you don't stress the joint and grind up your knee cartilage. Do not, for example, go to the gym and do leg lifts against a weight unless your soccer muscles are already decently developed and you know how to do the lifts without locking your joint or straining your tendons. Do not do stepping exercises or stair climbing, either, not to start with. Squats can be counterproductive, too, if you've been using your quads instead of your soccer muscle - they can just make the quads even stronger, exacerbating the problem. One thing you can do as a first step is to sit in a chair and put one leg out in front of you, then practice tightening the soccer muscle and holding it as tight as you can without hyperextending your knee, for a few seconds. Relax, repeat. Switch to the other leg and repeat. Build up gradually to longer durations and more repetitions. Work more with the weaker leg, if one is weaker than the other. If you can, try to get the soccer muscle to clench while keeping the quad relatively relaxed. This can be done, it just takes a little practice. This is something you can do while watching TV or sitting on a bus or whatever. Don't fatigue the muscle or make it incredibly sore, just make nice steady progress with it.
Once you have a little bit of muscle on the knee, and it's used to the idea of clenching and relaxing, try asking it to work while you're walking. You would be amazed how many people don't use anything but their quads when they walk - hyper-exaggerated example (sorry for the stereotype!) is the "cute little girl" shuffle that some young women from China and Japan do. So, when you are walking, see what combination of muscles you need to use to squeeze-relax-squeeze-relax with your glutes, your quads and your soccer muscle. How much do you have to flex your leading foot to get your soccer muscle to engage? How much do you have to ask your back foot muscles to make a little "spring" to propel you forward onto that front leg as it hits the ground, flexing slightly and engaging your soccer muscle as you shift your weight onto your front leg? Let your arms and shoulders swing freely; let your chest and core muscles move and work with your legs to maintain your balance. It feels really weird if you're used to the idea that nice girls hold some parts still when they walk, but actually things flow more smoothly and discreetly, if athletically, if you let all the muscles work together as nature intended.
Once you can feel how the soccer muscle works in walking, you can start jogging. Be sure to reach out with that front leg and let it flex to work the soccer muscle - no piston-style, leaning forward, hunched shoulders, rigid arms. The objective is not to go fast, it's just to work that muscle and work on balance and getting all the muscles to talk to each other. Remember to breath and relax on top while the bottom is working - it's great practice for fencing, where you have to keep your arm loose while your legs are working.
Make sure that you have orthopedic arch supports in your shoes if you need them. A lot of women have "X" legs, which can really make it hard to use your muscles right, and inserts can correct the problem and make you suddenly feel like Ms Amazing Athlete. They are sort of like contact lenses for your feet - nothing to be embarrassed about, and lots of people need them. Custom-made ones from a skilled professional are best, and are a good investment because they make training more effective, but anything that offers support will help. Of course, you might be lucky and have good alignment to start with
So, if you've once gotten a bit of the soccer developed, then you can do squats and fencing footwork with lots of lunges to further strengthen it. Get the back leg soccer muscle strong by doing more lunges on that side - i.e. doing footwork left-handed if you're a righty or right-handed if you're a lefty.
That's about all the advice I remember from my physical therapist, and my own experience. Hope it helps!
Glad that things are going well with your training. I'll be over here on this side of the pond doing all the same stuff.

Drop me a line if you discover any specially good drills or techniques!