I think for this race - You should share your concerns with your coach and ask if you can turn that 24 mile trail run into a shorter distance.  If you feel 2 hours or 3 hours would be better - Tell him why you feel that way.  Ask him what training benefit he feels you will get from the 24 miles.  Share that you are afraid you will not properly recover in the 2 weeks leading up to the race and would rather be fully tapered.

Get a dialogue going - At least try and understand what purpose he is shooting for.

I do a lot of really long runs - 30-40 sometimes 50 miles in training.  I find that it takes me about 4 weeks to get a training lift from these long aerobic efforts.  I know the prevailing wisdom is an average of 10 days to get the benefit from a workout - I follow that for speed work.  But with dozens of long aerobic efforts - It is always a minimum of 3 weeks and almost always 4 weeks before I feel the lift.

The longest I have run within 15 days of a goal race is 3 hours.  3 hours at an easy effort is about = 100-120 minutes at a moderate effort.  That is why I suggested that if you do the 24 miles - Do a super easy effort.  Mix in a fair amount of walking.