John Dahlz recently lead out of the water at Ironman Arizona with a 46:56 swim. In talking to John post-race, we asked him to share his favorite swim set for 2.4 mile prep. John says, "My favorite workout for ironman training is 800 repeats on a tight interval (ideally your threshold base interval)."
4 x 800 on 9:20(short course yards) descending 1-4 with 2 x 100 IM on 1:30 as recovery after each 800, so it's really 4 x broken 1k's.
"I like 800s because they are long enough that you can get into a rhythm, but short enough you do not get bored. The 2 x 100 IM's break up the set both mentally and physically, but still maintain your aerobic heart rate throughout the set. I think descending the intervals is key for the 800s because it gives you the confidence that you can get through a 4k main set feeling strong. Plus with the IM thrown in, the set goes by pretty quickly and with a w/u and c/d you can get a 5-6k workout in in less than 1.5 hours."