You are still young enough, but hurry!

As far as your enlisted performance...obviously they would expect officer candidates to be able to perform well if serving as junior enlisted.

If your only documented problem was weight/fitness, that's an easy fix...get in really good shape. I mean so good that you look like a navy seal...that will send all the message that needs to be sent when you interview. A picture is worth a thousand words...

If you had other documented performance problems, you will have a harder road. You will have to explain that you have matured and are more motivated. Good grades and obvious physical fitness will hopefully send the message that you have done a 180. If you have time, do some triathlons or similar activities and document it.

If you were actually separated from AF due to performance or fitness issues and it is documented on your DD-214 you are pretty much out of luck. The economy sucks right now, they have plenty of applicants as it is. Look up your separation code at the bottom of the DD-214 if you're not sure.