I’m already a very good swimmer, even as portly as I am right now. Many years of swimming, in my youth, as a swim-team guy — endurance swimmer, not a sprinter.
So Triathlon might be neat to work on. Never been a runner though. Forced to, in the NY Guard, but I hated every sloggin stride.
If you're a good swimmer you've already conquered the most difficult part. Lots of people (including myself) start out hating the run but it eventually becomes their favorite... it can be very therapeutic.
If you ever decide to look into it a little, this is the place to go to learn everything you need to know to get started: beginnertriathlete.com - lots and lots of people there that got off the couch, put aside the booze and tobacco and found a whole new lifestyle (including myself - but let's not get crazy, I still reward my self with a Scotch or IPA once in awhile).