I think there are several issues with your approach for improving power on bike:


doing the same workout stresses the same aerobic pathway, since there are several ways your body can burn fuel it is worth exercising all of them. This means doing intervals of different length with different rest periods. Example: 3x(12+6) min on and off. The on part is similar to what you can do for 1 hour and the off part is half of that. Another example: 3x(3+3) min on and off where the on part is what you can do for 5 minutes, off part is half of that.
the intervals do not seem to be anaerobic, because of the short recovery period( anaerobic intervals need longer recovery periods ). They are either Vo2Max intervals or Threshold. If they are Vo2Max I think the workout is too hard, usually accumulating 6 to 9 minutes at Vo2Max in a training session is too much, if they are threshold I don't think the approach is very good, better go with a longer interval period since threshold power can be usually sustained by riders for 30 to 60 minutes.
if you want to endure in long events/rides you need to do longer sessions to have the endurance
heart rate on an indoor trainer seems to be artificially high for me, I think it is because of the poor ventilation. Also power on the trainer is much lower then going outside, so pacing with heart rate on an indoor trainer seems to be a bad idea at least this is my experience. Suppose your 170bpm is upper tempo power, on an  indoor trainer that might be actually upper endurance because of the artificial high heart rate.
for effective mountain biking you also need riding skills, these are not developed on a trainer.


I would try to look at the requirements of your specific events and target those, also try to do longer sessions on the trail to build the endurance needed to complete this events.

An alternate way is to get a generic mountain biking endurance plan and build on that, it won't be perfect but I think is much superior over what you have now.

Also, try to ride outside if possible, the workouts are better and you will enjoy the training much more.

Is my interval training routine effective for mountain bike training?
What is the relationship between the reach of a RF signal and its frequency?

What I mean is: If power is kept constant, should I use high or low frequency waves to get a better reach? Why?
