W' does not tell you how hard a workout is.
A 20-minute workout close to FTP is very hard, or very, very hard. W’ won’t tell you anything. Even though over-unders like these below have an anaerobic component, they are primarily aerobic. W’ can not predict the difficulty in these. It’s probably obvious, but we call them over-unders because they are a little over FTP and a little under. You can play with the percentages above and below as you like.
Over-Unders – Most top coaches recommend starting on the easier side of hard workouts and working your way up. Micro-damage creates gains much faster than too-hard of efforts. Too hard of efforts frequently requires too much recovery which slows your progress. Really, about a total of 20-30 minutes of work is about as long as top coaches recommend or that sane people complete (including the pros). These workouts below are a total of 9-minutes. Start with these, clone them, and each week make the work-intervals a little longer.
A little bit harder but another great starter.
You would likely perceive this as a little harder workout because of the shorter rests, but still a great place to start.
Here is a link to the workout