I started off as an 18 stone front row forward, so not far off what you are now. Just did it bit by bit. 6 years, a London marathon and 5 Ironmans later i'm 12 stone 7lbs and a lot happier.
Important thing to remember is that you aren't going to change over night. Set yourself realistic goals and have the long term vision to see where you want to be in a few years time.