Dev you make a great point re: faking a 5k... I guess this is true.
But, the reason I like 5k is that is shows your *potential* to run long, not your actual ability, and I think your IM run split is much more based on potential. This talks to the issue of athletes who go out and do a marathon in prep for an Ironman just to "check" to see that they can do it. As we all pretty much agree, this is the worst training you can do for an IM. Here's why:
Bike pacing and effect on IM run
Bike nutrition and effect on IM run
overall weekly run volume and effect on IM run
weekly long run and it's lack of effect on IM run
as I said before, you're IM run "power" is based on your FTP, which is an artificial and arbitrary number for a certain length of time. I'm saying that your 5k time is the "FTP" that your IM run is a function of. Think about it... how embarassingly SLOW is 8 minute miles? That's horrible, yet if you split 3:30 off a 5:15 bike you're a stud. See what I mean? Do you really think slogging along at 8 min miles had anything to do with a 10k time? Really, it had to do with your 10 hour power, or CP 10h if you prefer. your 10 hour power is probably, for the trained athlete, 95% of 5 hour power, which is 95% of 2.5 hour power, which is 95% of 1.25 hour power, which is 95% of you get the idea.
So, if that were true, do you want to spend all of your time noodling around in training at your CURRENT 10 hour power or less, or do you want to invest your training time now to increase your FTP power levels to go faster later on race day?
If your 5k time is slow, or your 20 minute bike power is slow, don't expect to go at 85% of FTP on race day for an IM and expect to be able to run after. If you spend all of your run time running at "IM pace", effectively de-training yourself and lowering your run FTP, don't expect to be able to continue to hold 225 watts after the bike deep in to the marathon leg.
So, in the end 5k is ability, 10k is did you do the work, but really it's easily predicted, if you did to the work, from your open 5k time, and vice versa. I'm going under the assumption, that for me, I am going to do the work and therefore I'll start my race day execution planning from the open 5k baseline, not how slow did I run on my 4 hour long runs.
Eric Reid
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