I've found that it works this way for a while, but as fitness rises getting that same bone deep exhaustion gets quite difficult (at least without spending hours on it). Is there a way to switch things up and keep that going long term?
Higher intensity interval style exercises can get you there no matter your level of fitness. If you’ve never tried sprint intervals on an assault bike I’d recommend giving those a shot.
What's working for me right now is 'running' up hills. Running not in the 'fast' sense yet but it works.
I found a nice route, 7-8km, 250m up and the same down in three climbs and descents each and will keep running it for now. As I can always increase speed to make it more demanding, it will be fine for a long time most likely.