In reply to Daniel Heath:
It's pretty simple mate.
You've been out of school a few years; so no enforced exercise.
You've been driving a few years; so have lost all the exercise from walking about.
You've been on the piss a few years; all those pints and kebabs are adding up.
I'm guessing you're either working or in a more time consuming form of education, so less time to be out and about playing sport etc.
All this will have a huge impact. It probably seems like not much has changed since you were 16, but it really has.
I'm 23 and found myself feeling the same as you a few years ago, all it took was a little focus and determination to get my diet in order and make time to be active. I'm in good shape now and love exercise. Friends of mine didn't recognise how much their lifestyle had changed since school, and now look like 40 year old truckers.
Just don't get complacent and it's easy to stay in shape, not to mention being good fun. Surrounding yourself with people of a similar mind-set is very helpful.
Don't despair, keep your chins up!