I started sailing in high school and I was fat and out of shape but I was good and I was told I could have been very good. The joy of sailboat racing as the comment above made very clear, it's a brain game. If you're interested you can learn to sail, and race, pretty much anywhere with water, most places have some club or group that is racing Lasers, it's a good place to learn, how fit you need to be depends how far you want to go but it's not the primary factor in being good as far as I can tell (I'm an ok sailor these days, but some of the folks in here are clearly very good).