having spent most of my life either playing football or riding a bike, I find that my legs tend to sink making swimming difficult
Legs sinking is nothing todo with muscles or body shape – lots of people make this mistake. It is poor technique and lack of balance in the water. If your legs sink, it is because you are not getting the top of your body pushed down into the water enough. If it was true that heavy legged people couldn't swim, triathlon wouldn't exist.
There is a great book called "total immersion" which teaches how to fix this so that you don't suck at swimming. I only suck a bit at swimming now.
Joe