I heard in some podcast I cannot recall that combining hard mental work with physical training simultaneously can create a stronger adaptation specially on the capacity to sustain pure suffering.
They mentioned this test (link below) that can easily be done in the trainer.
In simple terms you are shown a word red, blue, green or yellow which is displayed in a specific colour that not always matches the word meaning.
You have to press a key that relates to the displayed colour not to the displayed word.
So you need to be constantly solving a cognitive conflict as fast as you can.
This seems to train the same mechanisms that one faces when physical suffering gets to a level that your brain also needs to manage the conflict between deciding to go on or deciding to protect yourself making you slow down.
Does this make sense?
I tried it a couple of times while doing some sweet spot intervals and I felt miserable to an extent that I never ever wanted to try it again.
But maybe this just means that the stress load was being hugely amplified so I should embrace the pain and get on with it to get the best adaptation bang for my time invested buck.