A lot of people ask me, "What's the biggest difference between training for running and training for cycling?"
The simplest answer is that an athlete should have so much more opportunity to suffer on the bike.
If a runner's training not limited by time, motivation, or illness; what is it limited by?
It's fatigue and the ability to recover from previous workouts. If recovery was not a limiting factor, he could go out and run 3+ hours, or complete intervals at 5K race pace day after day without the fear of injury or exhaustion.
What about the cyclist? Yes, fatigue is a real thing for the cyclist; but mechanically, cycling is very different from running. During cycling an athlete experiences very few eccentric muscle contractions.
Meanwhile, the runner is constantly subjecting his quadriceps, hamstrings, hip and plantar flexors to impact forces and eccentric muscle contractions. These eccentric contractions cause muscle damage, in turn causing muscle soreness. Muscle soreness may alter stride mechanics (Tsatalas et al., 2013), decrease economy (Baumann et al., 2014), making running fast more difficult and potentially leaving runners vulnerable to injury.
So, the muscle damage incurred during running puts limits on the volume of work that can safely be completed by the runner.
To demonstrate my point, here's an 8 day block of training aimed at increasing maximal sustainable power output (MSPO), for the cyclist:
Day 1 - 6 x 5:00(5:00) @ 110% MSPO
Day 2 - 5 x 2 x 3:00(90) @ 115-120% MSPO, 8:00 between sets
Day 3 - 5 x 4 x 90(60) @ 120+% MSPO, 6:00 between sets
Day 4 - off
Day 5 - Easy 60-75:00
Day 6 - Easy to Moderate 90:00 - 2 hrs
Day 7 - 6 x 5:00(4:00) @ 110% MSPO
Day 8 - 5 x 2 x 3:00(75) @ 115-120% MSPO, 6:00 between sets
This is nothing outlandish for many cyclists, but outside of Canova's one or two day, "special blocks." Have you ever seen a runner complete a block of training like this? If you have, I'd love to hear about it. The closest thing I can think of is the training of Brenda Martinez from Joe Vigil (HERE). Even that schedule on the bike would not be very daunting.
You'll never see professional runners complete the same amounts of volume as professional cyclists. Cyclists will log 3-6 hours in the saddle a day, even the best marathoners will not equal this volume. And you'll never see a running stage race like that of the grand tours - simply because no runner could survive 3 weeks of 3-6 hours of racing/day - not at the same intensities (I'm not talking about some multiple day charity jog).
So, why then don't we see more block training from runners? The technical answer is eccentric muscle contractions. But simply put, it is the product of common sense and trial and error - the risk of injury is too great.
I will not deny that there could be potential in block training for runners, but with careful manipulation. I think there are components athletes and coaches can transfer across disciplines, but you can't simply mimic cycling training if your a runner and vice versa.