why would it it be "above sustainable pace".
Humpty Dumpty smiled contemptuously. 'Of course you don't — till I tell you. I meant "there's a nice knock-down argument for you!"'
'But "glory" doesn't mean "a nice knock-down argument",' Alice objected.
'When I use a word,' Humpty Dumpty said, in rather a scornful tone, 'it means just what I choose it to mean — neither more nor less.'
-- L. Carroll
High-intensity long-haul efforts can require months of recovery, or be permanently crippling.
The 100m world record is 9.58s. The world-record marathon pace, 17.2s/100m. The Race Across America 3,000 mile bike race record, a sustained speed of 12.57 mph, or 17.8s/100m.
The specific physiological mechanisms for physical and mental exhaustion differ, but the general principles are similar: metabolites, waste products, side-effects, and damage accumulate. Absent a period of rest and recovery, these will eventually prove damaging.
In sport training, there is a carefully calibrated set of activities and rest and recovery, ranging from in a given motion (power and recovery stroke in running, cycling, swimming, rowing, or virtually any other action), to exercises, efforts, sets, workouts, seasonal, and lifetime scheduling.
Speed, skill, and muscle aren't created on the track, in the pool, on the road or trail, or at the gym, they're created in bed, when you're asleep, during recovery, given adequate nutrition. Training is stress, but a stress that's calibrated to trigger a conditioning response.
If you don't get rest, you'll simply break down.