For a good discussion of this topic get the Triathlete's Training Bible by Joe Friel.

Essentially, by working the aerobic system you have been training your body to become as efficient as possible at an aerobic HR. Since you are not working the sprinting speed, you are losing that skill/ability. Losing this skill is not necessarily a bad thing. If you are focusing on an IM, you have no use for sprinting skill.

The results you list are exactly what you should expect from long aerobic training. One of Friel's tests is to run a 5k at 10 beats less than LT - in other words solidly aerobic. Before starting aerobic base work I could complete this 5k in 28'. After 1 year of base aerobic work I can complete 5k at the same HR in 22:15! Yet, if I try and go over LT I'll crash hard.

There are a lot of good questions in here. First, this philosophy (Dr. Phil Maffetone, Mark Allen, Mike Pigg, et al) of training in ayoour low aerobic zone is effective at improving overall cardiovascular efficiency at lower levels of exertion. This is the level of exertion employed at ultra distance (Ironman). It uses the "specificity" principle of training: You are training for a long duration, low intensity event by doing low intensity, long duration training. Doing this for a year gave me a "bomb proof" aerobic base that enabled me to do 100 mile + running races and super long adventure races. But I couldn't run a 36 minute 10K right now to save my life. As a matter of fact, I doubt I could break 40:00 at 10K right now. Since my primary focus is running 8:30s for 26.2 miles after being on the bike for 5:20:00 I really don't care if I can maintain 180 bpm for 79 or 80 minutes. I short I wouldn't be too worried. It sounds like the LSD training is having an effect- it is improving your efficiency. There is the cost of some top-end performance though, but you can get that back eventually if you want to. Personally, I'd rather go under 10 hours at Ironman than go under 2 hours at an Olympic distance race.

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Tom Demerly
The Tri Shop.com