IMO, base training is the most important aspect of training since everything is built on top of it. If you're feeling tired during your base training something isn't right. My LT is 151 BPM so during this base period I'm keeping my heart rate below 141 which is my max HR for zone 3. I have done the vast majority of my workouts in an easy gear and high cadence. My favorite base building workouts are the aero base builder spinerval DVD's. I have all 4 of them and do them at least two nights a week. I have found the variety of the workout good enough to keep me from getting bored with them. Coach Troy does throw in some big gear grinding to keep things interesting as well. On the weekends I'll get out and ride on the road. I've found it's easier for me to manage my HR during the spinvervals workouts than out on the road.

This past weekend my buddy and I did a 25 mile ride and we hammered the 2nd half of the ride. In the past I've had difficulty keeping up with him but this past weekend we were wheel to wheel so I know my base building is paying off.