You simply cannot put in enough mtb miles to properly train. Your body will bet beaten up more (and quite possible injured), but you will be in no better physical shape. You need to ride the road if you want consistent training. ALL good MTB XC racers spend about 4 times as much time on the road as they do in the woods. This is done for the reason stated above.
So, my advice is to cram youself into some spandex, and get the heck out on the road to train! You will be SO MUCH more faster in the dirt if you do, and who knows - YOU MAY ACTUALLY LIKE THE EXPERIENCE!
