Here's a more simple premise:- of two riders (all other things being equal) who would be able sustain a 200 watt output for longer - the rider with an FTP of 250 watts, or the ride with an FTP of 350watts?
This is like the that Verizon guy talking to the kids: "Is it better to go slow, or ... is it it better to go FAST?" Luckily, it has gotten more productive since.
Here's another anecdote
, this one from 6-time Ironman champ Mark Allen:
My coach would give us workouts that were designed to push us to our limit every single day. I would go home dead, sleep as much as I could, then come back the next day for another round of punishing interval sets ...
So thats what I did. Every run, even the slow ones, for at least one mile, I would try to get close to 5 minute pace. And it worked
sort of. I had some good races the first year or two, but I also suffered from minor injuries and was always feeling one run away from being too burned out to want to continue with my training.
Then came the heart rate monitor. A man named Phil Maffetone, who had done a lot of research with the monitors, contacted me. He had me try one out according to a very specific protocol. Phil said that I was doing too much anaerobic training, too much speed work, too many high end/high heart rate sessions. I was forcing my body into a chemistry that only burns carbohydrates for fuel by elevating my heart rate so high each time I went out and ran.
To keep my heart rate below 155 beats/minute, I had to slow my pace down to an 8:15 mile. Thats three minutes/mile SLOWER than I had been trying to hit in every single workout I did! My body just couldnt utilize fat for fuel.
So, for the next four months, I did exclusively aerobic training keeping my heart rate at or below my maximum aerobic heart rate, using the monitor every single workout. And at the end of that period, my pace at the same heart rate of 155 beats/minute had improved by over a minute. And after nearly a year of doing mostly aerobic training, which by the way was much more comfortable and less taxing than the anaerobic style that I was used to, my pace at 155 beats/minute had improved to a blistering 5:20 mile.
That means that I was now able to burn fat for fuel efficiently enough to hold a pace that a year before was redlining my effort at a maximum heart rate of about 190. I had become an aerobic machine! On top of the speed benefit at lower heart rates, I was no longer feeling like I was ready for an injury the next run I went on, and I was feeling fresh after my workouts instead of being totally wasted from them.
There's a place for intervals in Allen's method (click the link for his full approach), but the bulk of training is definitely low intensity.
Incidentally, it was REALLY hard to ride that slow when I first shifted to z2 training. I'd be going up climbs, get passed by the 250 lb woman, or the 80 year-old grandma. It felt humiliating as I did my best to keep my HR down. That transition might've been the hardest thing about going this route. Luckily, you get faster.