My recent open half marathon pace was 7:42/mile. The conditions were ideal at 42Â° with low humidity. Based on that, I had hoped to run around an 8:00 pace and no slower than an 8:30 pace in the Gulf Coast HIM.
I averaged 9:00 at Gulf Coast and faded in some of the later miles to the 9:20s. I felt great when I got off the bike and started at a sub-8 pace. I had to consciously dial back to 8, and I held that for the first four miles. But then I faded very fast. I never walked (except pauses at most aid stops for water & calories), but I could not muster a fast pace.
I don't think I over-cooked on the bike. I rode at 75% FTP and easy RPE expressly to conserve for the run. My average HR in the run was a few BPM lower than my much faster HR in the open half marathon I referenced.
Gulf Coast was very hot, so maybe it was just that. But, heat alone doesn't seem like it would slow me down that much. And, many others ran fast.
I don't think it was hydration, because I drank four or five bottles on the bike and I did pee during the run.
I thought I had decent nutrition, but maybe not. I took in around 700 liquid calories on the bike and a few small cups of Gatorade on the run.
Is it run volume? Not enough bricks? What else causes a big disparity between HIM run and open HM run paces?