It's great, I'm in the summer break now trying to bulk up from wing to prop.
Just head for a club and join in training. Most clubs have a real amateur level for old people, fat people and newcomers (and I fit at least two of three categories, so I was fine).
And rememebr to bring your drinking shoes with you. Nothing better than a good match followed by a few beers with the opposition.
Though it WILL hurt, especially in the first game or two. But that's half the fun. If you don't end up limping off the pitch covered in mud you haven't had a good game (or you are a back...)