Actually I think it is a good thing to work out even with PKU, if you increase your intake of formula, you will be able to roll with it pretty easily, and it will probably help you sleep as it would use that extra energy you have. I actually weightlift and I am doing great with it. I just treat the formula as I would a protien drink and slug it like its water.
If I were you, I wouldn't try to fit in with the crowd, granted you have PKU but you don't have to define yourself by it. You don't need disability to be successful. Think, you're the man, and you can make it happen anyway.