Okay this is going to be kind of vague, but perhaps someone here can help me out

I don't have a power meter but I have an HRM and a cyclometer that measures cadence.

I did a 3:20:00 ride yesterday and I noticed something strange that I've noticed before. During the last 40 minutes I had a "brisk" (~160bpm/80%) interval planned at 80-90rpm. When it came time for the interval I started pedaling harder and shifted up to keep the cadence at 80-90rpm. About 20 minutes into it I noticed I wasn't breathing as hard as I do at the beginning of the day, but heart rate was still up there, RPE was constant and about where I thought it should be, speed and cadence were staying level. I pumped it up a little bit, thinking that something must be wrong. Speed went up, cadence went up too (didn't shift), heart rate went up to about 170bpm/85% and I started to feel the burn, just barely. Breathing moved slightly, but not nearly what I thought it should be based on an interval earlier in the ride. It felt like great news!

My question is whether aerobic conditioning can improve slightly from the beginning to the end of a 3hr ride. I recall thinking that if I was performing this well at this level of breathing, perhaps if I forced a higher rate of breathing I could go even harder. (Now that I think about it though, lactate doesn't depend on how oxygenated the blood is. Or does it?)

The other side, of course, is that heart rate and RPE were high because I was hitting fatigue and limits, and breathing has nothing to do with it. But since speed and cadence were coupled with RPE and HR, I thought not. I was heavily carbed-up though, and bonking was a long way off. I guess there's no way to know without a power meter, right?

Thoughts?