Back to the thread topic, and I do love this quote. Spot on, with a caveat though ~ if training has to hurt (or be very hard) to give results, then how do you differentiate between fatigue that requires a day off / gentle session, and fatigue/less than 100% recovery that means you can still train? I rarely train at 100% recovery (as that would imply race-tapering and missing too many sessions).
I do call my training 'adaptive', along lines Chris says here. As different parts of the body may be at a different (subjective) state of recovery, I may also choose which hard session to go for depending on how my body is feeling. In the past especially, I would run very high intensity uphill treadmill intervals, which I could manage while calf or hamstring strains healed. Also, I do not plan days off, but I do take a rest day when one is needed.
It's been interesting reading this thread because
(a) when I showed a coach my first training log a few years back, for a discussion about the interval work I had developed, he asked How do you get faster when so many of your training runs are steady?! Not in any case being widely read, I was slightly indignant and wanted to say that being marked steady only meant it being 2 or 3 minutes slower than a hard effort! And indeed I was setting PBs even on runs marked as steady!
(b) inspired by
I found an article http://www.sportsci.org/2009/ss.pdf about the merits of high-volume 80:20 training vs low volume, predominantly HIT based training. I think this might be a better way to summarise discussion than the thread title? And the upshot is, my steady runs are rarely slow enough, according to this! (The article is arguing against the modern idea that a top athlete can get away with low-volume, all high-intensity training, although admitting that for many regular athletes it may be the most effective use of time).
My annual strategy is partly worked around winter, as my metabolism struggles without sunlight. (Take Sunday: it was sunny, I had been out training twice (steady) by 1:30pm and felt so bouncy that I very nearly went out a third time, although opted for weights instead on this occasion; but come yesterday morning it was dark and gloomy and I felt unsteady trying to walk around the house! And I took a day off).
So, after a late-autumn break, I start training closer to once than twice a day during December/January, but work on intervals (and make my continuous runs harder) to bring up my max speed, vo2max, and aerobic capacity to a more acceptable level. This plateau's after a while. Also, thinking about technique is good before building the volume.
Then, and I will do this suddenly, I will double the volume by doubling the number of sessions, obviously running a lot more slowly for a few weeks. Multiple sessions is the safest and most effective way for me to do this; long runs just make me tired in a bad way and can encourage bad technique. Often, I will run the same course with between 1-5 hours gap between. After a few weeks of this, my times start to match the times I was running once a day, and I'll gradually build up the interval work again. Also, I'll start to run longer sessions, effectively combining 2 sessions into one. I see the big adaptations in this block as being metabolic (including appetite and digestion / glycogen resupply as well as fat-burning).
So my approach first builds up intensity on a moderate volume, rather than putting in hours of slogging. However, while the volume is being increased suddenly, my legs will feel tired or heavy going into a lot of the sessions. As long as it's just legs that feel tired, and not general fatigue, this is OK. But this goes a little against the top quote from Chris: just for a few weeks in this training block, I need to go training even though my legs feel heavy. Of course, I'm not looking for an amazing interval session when I do it.
One last thought: fell-running is different! If you look at 80:20 training guides online, you will find that runners are told not to include hills in their steady sessions, as this will put you straight into the middling no-man's-land territory, instead of it being a low-heart-rate steady run. So, you can give up hills and train a lot on the flat? You will gather from what I have said that this is not quite my approach!