When cyclists are beginning threshold training, I begin with Zone 3 intervals. An explanation of training zones can be found here.
In general, the work-to-rest ratio for these intervals is 3 or 4 to 1. For example, after your warm-up complete 5 to 8 x 3 minutes working your way to Zone 3 heart rate or power (if you know your power training zones). Do one minute of easy spinning to recover between each work bout.
There are endless combinations of this basic interval concept.
In addition to intervals, you can do steady efforts for a longer period of time. For example, after a warm-up ride 20 minutes steady at Zone 3 intensity.
Depending on your training plan and your experience level, you can build your total Zone 3 work time to 40 minutes or so. While some coaches like 2 x 20 minutes as one of the final workouts, I prefer a single 20-minute set (recover for five minutes), then 5 x 4 minutes with one minute recovery intervals. The reason I prefer the last 20 minutes to be broken is because I've found it allows athletes to sustain a higher average power output for the work time.
The next step for time-trial training is threshold training. I use the same interval concepts covered in the Zone 3 section of this column, except intensity now moves into Zone 4 to 5a—ideal for the 40K distance.
For athletes doing time trials that are 30 minutes and shorter—or for cyclists focusing on 40K TTs as their key races--the next step is anaerobic intervals. The work-to-rest ratio for these intervals is roughly 1 to 1 and intensity is Zone 5b.
Here are a few of my favorite workouts:
Find a hill that is in the four to eight percent range and takes you over three minutes to climb when you are going at a good effort. Mark your start location and finish location.