Here it is: the longest big effort in any session in The Sufferfest library.
The plain and simple fact is that sometimes to stress your FTP you need to spend more than 20 or 30 minutes riding near your FTP. This session was designed with that in mind. While most of the climb is actually below threshold, there are enough surges above it that your body will constantly be at the limit of clearing out those nasty, painful metabolites at the rate you churn them out. Painsuckers, rejoice!
Keep in mind that most of your anaerobic energy systems will be very depleted by the time you hit the 30 minute mark. That means they won't be much help in contributing to power production. That's one of the reasons why the same power target five minutes into this climb feels much harder when it comes again at minute 30. Once those anaerobic systems are used up, all you have left is your aerobic system, which requires oxygen to create power. These longer sustained efforts are crucial if you want to really push your threshold to its limits.