I'm in my first off-season training with power. What's really incredible about it is how it can tell you that yes, you can do this for longer. I was doing 2x20-min threshold efforts a few weeks ago, and in that 2nd one I started to feel pretty bad 5 minutes in. And if I didn't have the meter, if I wasn't looking at the numbers, I would have been tempted to say "I cannot keep this up." But I knew that I can - and I held it through to the end of the effort.
I'm looking forward to getting race data in the spring, too. That should be interesting.
In the past I basically trained by "feeling" my zones. But that's pretty inaccurate - finding sweet spot, threshold, and vo2max just by feel. I know that this winter I've done a lot more high-quality work than I had in the past and I am hoping to see it pay off big time in the spring.