Just read this and to be honest you're an inspiration - to lose 5 stone is amazing, I know because losing just 4kg has been a struggle recently for me. I think you'll love doing the triathlon. Im also quite short sighted and find it doesn't matter in the swim as long as you can see in the general direction of where you are going. Don't worry about the bike - in my first triathlon I passed one person - when I got closer to them I realised it was an old gent out for a cruise on his sit up bike. As far as the run is concerned, if you have to walk it, so what? Hope you have a great day and enjoy the whole experience...
don't know too many people happy with weight/body image - the great thing about running and other sports is you start to see the body as a functional tool and forget (to an extent) societal expectations/pressures....
on another note I ran for the first time for about 5/6 weeks today - only managed about 1/2 mile but going to take it slowly to try and avoid any further injuries......still aiming for Cardiff 1/2 marathon in October and Paris next year though