Rather than building ftp by going easier than it, you can also build it by going harder than it. The added benefit there is you're sure to be working on some things you're certain to see in the kind of races you're doing on a regular basis. Stringing together a bunch of interval sets until you're getting in the kind of hard volume you'd see in a hard hard race. I've never soloed for 20 minutes in a crit. I've been in breaks that lasted more than 20 minutes, but obviously those are on and off affairs and more on and off as tactics dictate. Hitting it hard, and recovering fast. Most efforts that are solo are 3 minutes or less. I soloed the last lap labor day. That was a mile. 3ish minutes? I'd have to check the file, but I'm guessing there wasn't much Z2 in there.
That road race I didn't win last season? I wasn't winning it even if I had spent the whole winter preparing for it. And I the reason I didn't win wasn't because I couldn't ride for 3 hours in Z2. I'm losing to guys who are better designed for those kind of races.
One really needs to be an outlier on the lower end of the scale to be limited in a crit by lack of base. It isn't what causes guys to fail to make the winning move.