I'm a very fit triathlete one month away from my first Ironman. I'm strictly following Don Fink's training plan, which includes "interval training" in HR zone 5, or 90%+ max HR. (Fink actually calls it "zone 4" for some reason.)
The workouts used to involve more traditional intervals: bring your HR up to 90% for a few minutes, then rest, then repeat. But at this point in his plan - at the peak of the "peak phase" - he's asking me to do 12 minutes straight in zone 5 on the run, and 30 minutes straight in zone 5 on the bike!
I'm having a great deal of trouble with this.
How long should I be able to stay in zone 5? Am I simply not pushing myself hard enough, or did I just fall behind at some point?
Edit: on Fink's zone definitions, he writes:
Zone 4: 90-95% of maximum heart rate (primarily anaerobic training)
Zone 3: 86-89% of maximum heart rate (middle zone)
Zone 2: 75-85% of maximum heart rate (higher-end aerobic training)
Zone 1: 65-74% of maximum heart rate (lower-end aerobic training)
His example puts a 35-year old at 166-175 bpm. The vast majority of his plan is in "zone 2."