Good Luck, as the others have said Fitness will play a big part. Expect the infantry fitness level to be a higher requirement than that of the RLC. If you get through into the infantry there are a lot of things that you can do, ICE (Infantry Combat Engineer) what we used to call Assualt Pioneers can be good, RMA (Medical), MT (Motor Transport) and Signals amongst other things can get you decent transferable qualifications. You can also Re-Badge once you are in and potentially move over to either the RLC, AGC or REME and RE's as long as you pass their exams.
Whatever else happens give it a good shot, best effort all the way, but don't overly pressure yourself.