Ekiely,
Unfortunately, no one can tell the future. It greatly depends upon how far the epiphysis actually slipped before it was pinned, and as a result, how congruent the joint currently is.
Also, some patients with SCFE also develop a disorder known as chondrolysis after a slip. This is where the cartilage layer of the bone becomes thinner. In most patients, this reverses itself over about a year after the slip, but in a tiny few, it does not.
So, if you had a bad slip, the joint in not congruent, you had chondrolysis that did not reverse, then you may need a replacement in the future. But, if the slip was not significant (as in your hip that was pinned prophylactically), the joint is still in round, and you did not develop chondrolysis, then you probably will not need a replacement. Of course, most patients fall somewhere in the middle.
Regular exercise is actually good for the joints and the body. It helps to maintain range of motion, strength, body weight, etc. Overuse or extremely heavy impact activities may be too much.
So, you will just have to see how things go over the next few years. There is not really anything you can do about it now. Maintain proper body weight, exercise regularly, eat a proper diet, avoid smoking and excessive alcohol (both have been shown to affect the femoral head), and continue to watch how the joint feels.
Good luck.