How did players like Tim Henson manage to practice for 8 hours every day and what did their structure look like?

I just picked up the guitar again after a couple months and I decides to go guns and blazing and practice 2 hours of just technical exercises(scales, hand synchronization, finger independence etc.)

My routine was something along the lines of:

-warm up by going all the way to the 12th fret and back twice(1234)

-20 minutes finger independence

-20 minutes scales

-20 minutes hand synchronization

-20 minutes picking exercises

-20 minutes legato

I didn’t noodle around or practice any songs. I just wanted to focus on solely technical skill.

What I soon realized was that first of all, this was all pretty boring, and second of all that both my hand and brain were becoming tired(hand cramping up)

I always hear that some legends practice for 8+ hours, but I just don’t get how or what they practice for that long.

1. What would their routine look like? Do they focus more on their repertoire? Do they do even more technical exercises than what I mentioned?

2. How do they manage to not fry their brain and hands?