to me sustainable exercise is the key to a balance. Is it something u can do everyday or every other day post recovery. If doing that 500km can be a weekly event and no health damage then is okay.
Cannot lah...... my weekly mileage is around 250km and can feel recovery aspect is jialat, ie my legs never feel fully rested. 500km in 18 hrs (ie with timing) is no joke, you probably need to recover 3 weeks, from what i see on T100 triathletes. 500km, that's probably more than a full ironman in energy expenditure.
That is in addition to around 25km of running mileage.
My cycling sessions are daily, but vast majority pretty low HR / RPE. Basically just joy rides listen to music and see chio bus. But peppered in there are sprints/surges. eg I usually gun 1.5km at Bay East Garden....as to how many intervals i do like that, depends on the mood.
That is also probably why i have had no real progress in my running TT timings as well. (5 and 10k)
Insufficient recovery
But that is fine, coz no running race events etc anyway.