If you want to become a stronger mountain biker then you should consider spending a little time on the road. The road offers the ability to fully dial in your training to what you need to improve rather than going out and ripping through the trails, and the road is more practical in terms of time efficiency. You can train your systems in less time due to the increased specificity of your effort. In 1 to 1.5 hours you can train your lactate threshold, anaerobic endurance and/or strength systems. For example during a mountain bike event if you’re on a very long climb and your legs are frying and need the additional sudden power to push through loose steep sections or clear various obstacles it's those higher end efforts developed on the road that can push you through. Mountain biking is a very on-off the throttle type of racing and training exclusively this way can limit your endurance and power. Additionally, if you raise your lactate threshold or functional threshold power the higher your max number, the higher your 70 to 80% effort will be as well. Training a little more on the road is simply just fast tracking your physical performance.