Sounds like fun!!! The furthest I've swam was the IM swim: 2.4 miles (well, I probably swam more since I'm a terrible sighter) and that was fun, but tiring. I would imagine a 3.1 mile swim would wear me out- but in a good, fun kind of way. Is it wetsuit legal? Just wondering.
Have a GREAT time, and tell us all about it when you're done. I've never done anything like that, so I'd love to hear all about it.
Sorry you can't run yet, but you'll get there. In the meantime, this challenge is monumental and super exciting!!!!