CSugar,
Hi, you asked for advice, here it is!
I don't know where you read that cardio doesn't lead to muscle development, because it does. Not as much as weight lifting, of course, but people do firm up with cardio. Your inner thigh muscles are called the adductors. The purpose of the adductors is to stabilize the body during hip flexion and extension. Walking requires hip flexion and extension, so you probably have firmed up your adductors. I would guess that your hamstrings (back of thigh), quads (front of thigh), calves, and glutes (butt) have also become more firm from your walking, especially because of the incline work. I don't know if you are overweight, but if you are, fat could be hiding your muscles, but maybe not as much at your inner thighs.
Having said that, I can't tell from your posting what your goals are at this time. (Sorry, I don't remember if you strength train at all. If you do, you can ignore some of the following! ha ha) Are you wanting to lose more weight and/or firm your legs? What about your upper body? I would suspect that even though it's really great that you've been walking for 2 1/2 years, 5 days per week, you probably have hit a plateau. Any time we do the exact same exercise day in, day out, our body will adjust and after a while (about 6 weeks) it's not a challenge anymore and you stagnant at that level of fitness. (yes, even though you are sweating and becoming winded, your body needs to be challenged in order to improve, fitness wise.)
So, if your goal is to lose weight, I would recommend that first of all, you replace a couple days of your treadmill walking with a different type of cardio such as walking outside on hills, bike riding (stationary bikes are good, also) running, swimming, hiking, tennis, aerobic classes, whatever you enjoy. Your body needs a different type of movement, but if you just want to stick with walking, I think that 30 minutes per day of walking plus 30 minutes of strength training would be good. (don't know how much time you have to spend working out.) Secondly, like I posted before, strength training is very important for weight loss, and it makes us look so much better! You won't get big and bulky from lifting weights. Also, it's great for our bones, makes us look fit, and best of all, you get stronger! How about strength training a minimum of 2 days per week. (3 is better). You might want to focus on upper body strength training at first to balance out your body, as your legs have been getting the workout from walking.
OK, I know it sounds like I harp on strength training, but I am very passionate about working out, and I believe people can
not be truly fit without it in their workout program. ..down off my soap box!
Hope this helps in some small way!
jem