stephtu

Threshold usually refers to "lactate threshold", aka "anaerobic threshold", the point where lactate starts building up in the blood, you are producing it at a greater rate than your body can dispose of it, and will have to slow down at some point relatively soon after it gets to some level. A bunch of training programs base target heart rate zones off of this, determined by a field test of some sort (often some protocol involving time trial effort of somewhere in the 20-30 minute range), rather than off of max heart rate, which can vary considerably from formulas and is hard, maybe slightly dangerous to test outside of a lab w/ a doctor.

You can train just fine with a heart rate monitor, I agree if not racing a power meter is major overkill. I'm planning on trying Carmichael's "Time Crunched Cyclist" program, he gives guidelines for training both for those with power meters and those without. Basically depending on your goals (racing or just faster century type rides), he gives a training schedule for doing intervals of various durations at a particular intensity of effort.