Yay! I've got the first of my self-prescribed massively long workout under my belt, and now I'm one-third done. AH-HAhahahahahaha! (That's an Ironman joke right there, folks, is what that is.)
I kept an eye on the wall clock as I swam, and I hit 4,200 yards (Ironman swim distance, more or less—I think more) in about an hour and 14 minutes and the full 5,000 yards in about an hour and 27 minutes. I think it's realistic to assume that my race swim—with the cold and the chop and the crowd and the wetsuit and the FREAKING OUT and the lack of a big black line to guide my way—will be somewhere between those two times.
The other useful thing I gleaned from this was just how hungry I'm likely to be when I hit the first transition. It might be a good place for a couple of sandwiches, seriously. My son was making pancakes when I got home, and I was like STAND BACK, AND DON'T GET BETWEEN ME AND THE SYRUP.