Enjoy it for what it is and try not to get bored. Don’t hang up the mountain biking until you must. There’s no substitute for riding trails. I’ve even found that during build and special phases that if I’m not riding the mountain bike 2X (at least once) a week, I feel flat. The workouts in base are SO much different than what you do on a trail ride that I typically enjoy the contrast. When you get to build, it’s hard finding the balance of outdoor fun riding and nailing the plan 100% - I think most people benefit the most from a LV plan. The ability to do outdoor workouts (even just using RPE) is huge, if you can pull that off I was getting in the habit of nailing the workout and leaving a little time after to work on skills or even just taking the fun way back to the car. There’s a benefit from hitting the higher energy systems while in base training, might as well have fun with it too.
Kudos on doing the full blown 20 minute test too, I can’t remember the last time I did that version - it’s tough and shows a high level of mental toughness.