Hi Supercooper, first things first have a look around the site at the other forums, there's plenty of information for you. AJ's guide to fat loss and muscle building is a good start,
http://www.mfat.co.uk/forums/viewtopic.php?t=14159, or have a look at this one for a basic idea of a schedule
http://www.mfat.co.uk/forums/viewtopic.php?t=10683 obviously tone it to what you think you'll be capable of, and work from there.
Although both of those links take you to the RM part of the site, I'm not trying to influence you, there just seems to be a lot of good training information there.
I'm guessing that you've already looked at the Army jobs website
http://www.armyjobs.mod.uk . Find out about other careers on there, think whether you want to join the infantry, or go for a more technical or skilled role, you never know there might be something on there that appeals to you. Take down some notes on any questions that you might have, no matter how silly or stupid they may seem, and then take that piece of paper to the AFCO with you, you might feel a bit of a lemon pulling out a list of questions, but don't you'll feel even more of one if you get in there and your mind goes blank. The AFCO recruiters are there to help you.
And as for people pushing you into joining the Royal Marines or Para's etc, ignore them, I might be corrected on this, but from what I'm aware infantry can transfer to the Para's or become Para trained, and the corps have options to become either Para or Commando trained, but don't ever feel pushed into doing that, all services of the armed forces do a bang up job. So follow your heart, you're the only one who can make the decision at the end of the day.