It’s a both/and: accumulating more time in zone helps build FTP, while continuous time builds fatigue resistance and FTP.
The true iron-head would just start with 30min@90%, say, and then extend that to 40, then 50, then 60, and on, up to say 120min. That’s quite difficult psychologically, especially indoors, so breaking the time in zone into chunks helps accumulate the training stress.
But, the hard parts in races, and races themselves, don’t turn back into pumpkins after 15 or 20 or 30 minutes, so you have to work on extending the continuous time at a “sweet spot” effort, or better yet, extend the time you can over-under (like :30 sec @ 110%+ of FTP, then 2:00 at 88-90% of FTP, for 30 or 40 min…doable, but ouch).
From Tim Cusick, in Pez Toolbox:
Here’s a simple example. Say you do a lot of 2 x 20 minutes at tempo, sweet spot, or FTP training levels. This probably means you’re trying to get more watts each session, often turning tempo and sweet spot work into FTP intervals. I recommend that you focus more on increasing your time in those zones and let the power come up more naturally as you grow more fit. Instead of doing each 2 x 20 a few watts higher, progressively expand the duration of your time in that zones.
You could start at 2 x 15 minutes of SST and progress to 2 x 20 minutes and then then 3 x 15 minutes, which leads to 3 x 20 minutes of SST. I progress my athletes incrementally (often 1- to 2-minute increments) over the course of their base training, but there’s no reason to sit stagnant; I will rarely plan more than three workouts at the same time length before increasing the time demand. Just remember that your power numbers will be coming up as the time increases, so you’ll need to test and monitor other data to gradually move up your power targets.
Why give this a try? Results! Increasing your power duration/fatigue resistance is more likely to improve your results than adding a few more watts of pure power in the base training phase. How many times have you made the break and got into the lead pack only to be dropped or be unable to hold? You had the power, but you couldn’t sustain it. It’s time to change that.