Do you commute? Easy way to get the base miles in - if I just ride straight to and from work every day I get 150kms/week. Add in w/end rides (actually on the MTB) I get around 200kms thru winter.
Down under, we are getting into spring now, so starting to take the long way home couple times a week to increase the mileage and ride the MTB to work and go for a ride from work too.
Unfortunately, the boring hill reps and intervals have to be done to really get race fitness. Try to find a good route to keep them interesting - ideally a partner is good for this as the competition between you will give the incentive to keep it up i.e. "got beat him up this hill today", "gotta outsprint him this time", etc
the R&R/week off has to be down to let your body actually benefit - 4th week is good, can spread to 6th week as you get stronger/fitter, but if you feel crappy (not just tired, but like you can't do anything let alone ride) before that, listen to your boday and take a few days off, and make sure you eat well to get over the hump.
Also, make sure you don't do all your rides based on Road/HRM - it's easy to get disillusioned and bored, plus MTB skills need to be trained just as much as fitness does - too many people neglect them; no point being able to cruise up hill at only 60bpm @ 30km/hr only to go down at 180bpm @ 20km/hr cos you are crapping your pants at how "hard" the descent is cos you've got so used to everything being flat tarmac. sounds obvious, but...