Hi! I started using a bike as my main form of transport about a year ago, and I haven't looked back. At first, I hated it but now I look forward to it. I can't recommend it enough!
As to shaping your thighs, I'm not sure how much of that it will do. It's a great cardio workout so it will help burn calories and aid fat loss that way, and if you bike lots of hills or perhaps at a really high gear I guess you might gain some leg muscle strength. But! If you look at professional cyclists' legs, they have very specific muscles that stand out - their calves, and the muscles on the inside of your leg just above your knee. I don't know how much muscle definition it will add in your upper thighs though.
In saying that, I'm pretty sure cycling has done wonders for my butt! It's so much higher now! And I feel like it has made my waist more defined.
Keep going!! I promise those hills will get easier