Chris Carmichael has a pretty straightforward and easy to use method for figuring out how hard you should go for a specific type of work out. Here's what you do. Find a 3 mile fairly falt course. Ride the 3 miles as hard as possible twice with a 10 minute break in between. The average heart rate for both rides is used as the reference point for the work outs.
I've only listed a few of the types of workouts listed in the book. Carmichael has 17 different types of workouts but I've only listed a few of the workouts that I thought #1 heart rate based ( as opposed to some of the sprinting or strength workouts ) and representative of a certain level of exertion.
Foundation Miles
Foundation miles are the basic aerobic workout. Long and slow. To be done at 89% ( 86% if over 35 ) of the average heart rate of the field test. It's not obvious where this goes in the table so I put it roughly where I believe this kind of work out goes. I.e. with the other LSD type workouts.
Endurance Miles
Similar to Foundation Miles but slighter harder. 91% ( 88% if over 35 ) of the average heart rate of the field test.
Steady State Intervals
Steady State intervals are done at just below the Lactate Threshold, or at 92% ( 90% for over 35 ) of the field test heart rate average plus 2 to 4 beats per minute. Carmichael puts the lactate threshold at about 95% of the average field test HR.
Climbing Repeats
Climbing Repeats are similar to steady state except they're done on a hill and at a slightly higher HR then Steady State intervals. The target heart rate for these is 95% ( 92% if over 35 ) of the average HR for the field test. Which should be right at or just over the lactate threshhold.