It seems like what you're asking is how much more training do you have to do to go from preperation for sprint distance events to Ironman distance events. In my admittedly undereducated mind the central issue to Ironman training is getting ready to exercise continuously for 10-15 hours. Physiologically, that is a lot different than sprint races. Ask yourself, when was the last time you exercised continuously for 12-15 hours? At the most basic level, that is what you have to prepare for. And it takes some time to lay in an appropriate base and then build on that.

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Tom Demerly
The Tri Shop.com

As I understand your situation, you are quite fit, have IM experience, and want to do sprints and IM next year. I have found that doing sprint races in the early season actually helps my IMs.

In order to optimize your sprint racing you need to do speed work of a different sort than for the IM distance. So what I do when I want to do well in sprints while having an IM on the horizon is the speed work plus medium distance threashold work (8-10 mile threashold runs + 35-45 mile threashold rides). This makes me faster and powerful with plenty of endurance for the early season.

Then, about 6-8 weeks out from the IM I switch to IM specific training. So, if I understand your question, I think you can optimize your spint racing and need to allow 6-8 (i do 6) weeks to "switch gears for IMUSA"

Good Luck!

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David
* Ironman for Life! (Blog) * IM Everyday Hero Video * Daggett Shuler Law *
Disclaimer: I have personal and professional relationships with many athletes, vendors, and organizations in the triathlon world.

From the training programs I'm familiar with, the thing underweighted in IM training is the faster stuff. The last RAM winner said that the one difference that made his record-breaking ride possible was weekly (NOT Weakly!) interval training. To paraphrase him, if intervals were good for RAM distances, intervals are good for all distances.

Happy Sprinting!

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)