Swimming is the ultimate humbler. People can run fast and far. Cycle at speeds approaching moto-like power. But they can't swim.
Personally...I think the Ironman swim is too short. It should be around 5k in my humble opinion. Triathlon swims are a laugher in general. Only in the last year or so have we really seen swim times come down to truly fast speeds. This may be training (or high tech speed suits - which should be illegal) but finally triathletes are finally figuring it out.
*I believe swimming should be a test of who is most efficient and powerful in the water not who has the best speed suit. I guess I'm old school. Speedo and paper suit and shave it all.
Time In The Water = strength and speed. TITW is what Matt Keller said to me yesterday as I gasped for air in the first week of regular swimming for me in some time. Shoulders aching. Lungs exploding - it is time to get in shape.
Don't sweat the sets. Don't worry about hypoxic this or that. No flippers, fins, tubes or paddles. Just swim baby. First test set is tomorrow...
20 x 100 on 1:30 "best average" - it will not be pretty. 20 x 100 never is.