Last night was my third week of doing Joe Friel's M2 (muscular endurance) workout. Muscular endurance is related to your ability to sustain an effort at or just below your lactate threshold heart rate (LTHR). Fundamentally your LTHR is a level of effort you can sustain over a given period of exercise at a relatively high level of perceived effort. If you ride at a higher HR than your LTHR for too long your muscles fill up with lactic acid and you eventually crack. Typically cyclists determine their LTHR by doing a 30 minute all out effort. Your average HR for the last 20 minutes of the ride should be your LTHR. The last time I measured mine it was about 183 bpm.
The M2 workout is based upon riding successive intervals at a level of exertion that will bring your heart rate up or near to your LTHR. After each interval you do a 2 or 3 minute recovery. You then do another interval, etc.
Typically I do sustained workouts like this in the gym at my complex. I have a set of CycleOps rollers that I can ride on continuously without having to stop at lights, worry about hills, rain, wind, etc. When I start riding on my rollers I get a lot of funny looks from people, some concerned, some impressed, but once they see that I'm actually not going to fall off the rollers they seem to relax a lot more.
BTW, if you've never tried anything like these, it's quite the experience. Getting to the point that you can ride on these things is like learning to ride a bike (the first time you learned). They require an entirely different level of balance than simply riding on the ground. Consequently they do amazing things for your pedaling efficiency and smoothness. When you're riding on the ground you can choose to stop pedaling, turn, lean, etc. in order to balance yourself. On the rollers you need to keep yourself completely balanced at all times.
The first week of the M2 workout was one hour workout. That week I did 6 minute intervals with 3 minute recoveries. At the end of each interval I felt like I was going to fall off my bike.
The second week was a 1.5 hour workout. That week I did 8 minute intervals with 4 minute recoveries. I no longer felt like I was going to fall off my bike, was able to be much more consistent with my effort, and had to grind my way through 8 minutes of pain. Somehow the difference between 6 and 8 minutes is pretty noticeable to your mind.
This week it was a 2 hour workout. I did 12 minute intervals with 4 minute recoveries. Yikes! Last night's workout was pretty much the most pain I've ever felt on a bike. Every minute that goes by you're telling yourself something like, "okay only 11 minutes to go". Not so comforting when you realize just how far you have left to go. I did all 8 intervals however, so I was really proud of myself. It can be pretty tough to motivate yourself to do these sort of workouts when quitting is so much easier than doing the exercise.