I will politely disagree.

Anything shorter than 2x20s or under/overs at 12min or more if going by HR you may as well use RPE instead because the lag/reaction in real time in HR isn’t good enough to pace a pursuit interval.

Obree used a mechanical dumb trainer. But, a big but, he set it up so meticulously as to be able to gauge and track efforts.....like a power meter.

Tell me you can go do 3x3min sets within 10w each time using HR. No way for most anyone. Second set your RPE is gonna drive the actual output down as the anaerobic stores are further depleted than the first set and you ease up. The sensation is different but you’re driving adaptations through execution of that second set at the same power.

Robnol should have kept it more basic also for the topic.

I follow my work’s pro team on Strava, all of them seem to be running 10k workouts recently in off season. And doing speed skating and cross country ski.

It seems like a general health and balance of fitness topic got into a debate over how metabolic systems work and what workouts and sports do what.

Again, get Slowtwitch forum accounts and start reading.

Also, fwiw the IM bike leg cutoff time is about 14mph avg. The marathon cutoff is a 15:00 mile death march. Assuming you maxed the time on both the swim and bike. If a cyclist survives the swim, they could have up to 10 hours to walk the marathon.

If doing a 1/2 IM my last long TT bike ride was tempo at a pace that would leave me nearly 5 hours to walk a 1/2 marathon assuming a max time swim split.

I do race bikes but feel the IM finisher cutoff times are pretty generous given even the modest running I do with the bike training.

All that to say.....Don’t confuse things like barely finished a fondo or IM with someone competitive within it. Different worlds.