As runners or triathletes I'm sure we've all done this at some point in our lives. I know I've done this many times running at the track or at a 5k. I never thought I'd do this is on a long bike ride where the goal is to take it easy.
Two weekends ago I rode 120 miles with 90 of them being flat and 10 being rolling hills. Miles 100-115 were hills. One of them having the gradient profile as shown below.
It's a very short climb but a 7.3% average grade is steep for most cyclists. I felt perfectly fine on this ride until I hit the 0.57 mile point where the grade tops out. I went from feeling great to feeling as if I were going to die in about 30 seconds...I went over my limit. It took about 5 minutes to catch my breath and cool off. Needless to say, I tried to ride the hill too fast. I guess it's difficult to judge how quickly you can red line and how long it will take to recover after you've been riding for several hours. Once we got down the hill and back on to the flats, I was peachy. I decided to wait at the bottom of the two remaining hills (we made them all out and backs) to avoid a potential trip to the hospital. Like I said, I felt fine on the bike after catching my breath but then I crashed and burned a half mile into the run.
Last weekend I rode just about the same route except we hit the hills at 80 miles. I decided to take it easy on the hills this week and made it up all of them without any issue. I think my max heart rate was also 15 bpm lower and I didn't crash and burn on the run.
You walk a fine line in the later parts of long rides/runs. I'm guessing the same goes for later parts of the Ironman. I never realized how easily and quickly the point of no return can be reached until I reached it.