You can't really ride for 33 miles while anaerobic, you'd be toast after a few minutes. However, the harder you are going, the sooner your glycogen stores will be depleted, and yes, that depletion is what causes the sudden fall-off in energy levels after a couple of hours or so. You can delay that by eating as you ride, but most of us can only absorb about 60g of carbohydrate per hour (around 250 calories-worth) so you can't replace it as fast as you are burning it.

Essentially, how you train should be determined by how much time you can afford to spend on the bike. If you can manage > 10 hours per week, most of your riding should be done at endurance pace. Not slow, but steady - the sort of pace you feel you could maintain for hours at a time. If a couple of hours, total, is spent at intensities that are close to a maximal effort, that is plenty. Unless you live in a very flat area, on a long ride you'll typically encounter a variety of terrain, so if you go steady most of the time then attack the hills, you'll get the right sort of balance. Alternatively, if it is flat, go steady most of the time but put in some sprint intervals.

Incidentally, the 220 - age formula for max HR is bunk, and max HR isn't a very important number anyway. If you want to train on heart rate, you'd be better advised to do a test for lactate threshold HR as mentioned above, and work out your HR zones from there.