While do i feel like i am confessing a sin.
Each time i need to get back into riding or pick up some fitness my go to training trails are the hills near home. It is easiest way to gauge improvement - how far you can get up the hill before stopping/hyperventilating/spewing. when i started in March i had to stops around 5 times each trip on my hardtail. By July i was able to muscle the enduro up the hill. I was stoked to get up that fucking hill.
I find it easier to push myself to ride up a hill where the goal is obvious rather than sprinting on a flat track.
As an added bonus you get to bomb down the hill after you've scrambled up it.