However, I am wrestling with this issue this week. part of me wants to do a 3 hour run (at least 18 miles) for a confidence builder, but part of me is afraid it will toast me for the rest of the week. I will probably stick to the 2.5hr run (for me that is around 16+ miles) and then do another long run of 10-12 miles later on.
I am not an IM training expert by any means... but it sounds like you have your answer:
Last year I did a 3:20 run in training and then was dead for about a whole week, which I cannot afford to take the time away right now.
Good luck with the training!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"