Exercise can help, but extra skin that's stretched out isn't going to magically go back to where it should be. From my understanding, there are many factors that contribute to how much extra skin you will have at the end of your journey, such as how long you were overweight, genetics, skin elasticity, diet, exercise, etc.
That being said, I have managed to tone up my arms with exercise. It's not so much that the skin isn't there, it's just that it's not as noticable because the muscle underneath is toned up. One exercise I did a lot of until I was able to do things like push ups, were reverse curls (I think that's what they're called LOL). I got a heavy jug (like a full three litre bottle of whatever you have on hand in the kitchen) and grasp it with both hands behind your head. You elbows will be pointed to the ceiling and your hands and the bottle will dangle at your back below your head, then you pull the bottle up until your arms are almost completely straight and the bottle is above your head and lower it again. You can do reps of this with both arms, or alternating one arm at a time. This works the lower arm muscle pretty well.
Walking, running, swimming are all great for your thighs.
I hope these ideas help!