Some interesting & relevant reading...
Time crunched base training
Where do you feel you are presently with regards to your level of fitness? What is your age? What are your goals, what do you want to achieve and realistically how much time do you have available and are prepared to commit? I believe the latter will ultimatley decide the limit of your possible achievments i.e. having only 4hrs per week wont permit you to be competitive within a 100 mile mtb race.
You will need to incur some sort of training stress on whatever energy systems you are looking at developing and ensure enough recovery to premote development. Some form of progressive and structured training along with a nutrtion program is the best way to go here. There is a science behind all this but the art is knowing how to put it all together
Perhaps the most significant predidtors of performance in endurance events are having a high lactate threshold & VO2 max, although a high VO2 max may not be a prerequisite for performance in endurance events.
Think of VO2 max as an athletes aerobic potential and the lactate threshold as the marker for how much of that potential they are tapping. Whilst genetics plays a major role in a persons VO2 max lactate threshold is trainable and there are many ways to skin this cat.
FWIW last year over winter I felt my base level of fitness never really progressed and I seemed to lack the sort of endurance I was aiming for. I had plenty of frequency in my program, 6 out of 7 days I’d concentrate on building my aerobic fitness but with my low intensity workouts in the region of HR zones 1 & 2 and usually limited to 1-2 hours with an additional long weekend ride I realised that I didn’t have the volume at that intensity to promote significant physiological changes other than burning calories.
So my plan over the winter looks something like this, the majority of workouts will be high end zone 3 or tempo workouts varying in length from 3×20 min intervals to a full 2hr block whilst maintaining a high cadence, training in this ‘sweet spot’ area in between zones 3-4 has shown in some instances to give the best bang for buck when it comes down to physiological changes and development. Some long low cadence, high resistance hill type intervals will come later. I have also upped the length of my LSD (long, steady, distance) rides to 5-6hrs (weather permitting!)which include a few sets of short Vo2 max intervals, 1 or 2 of these during the week, sometimes back to back. I’m also going to try out 3 & 4 day training blocks with a complete rest day every 6 days and some short recovery rides in between combined with stretching and some basic core workouts this should see me through the winter months.