Yesterday I did a shorter ride than usual, and worked to stay in my HR Zone 2. I worked at a rapid cadence in low gear, trying to focus on smooth pedaling. It's surprisingly difficult for me to not break loose and try to push it to 20 mph for however brief a stint. But on a "rest" day that's a no-no.
So this morning I made two brief efforts to get up to 20 mph. I made it! It's getting easier =) Easier on my legs, that is. Such attempts still make my breathing fast and heavy. Even so, my heart rate didn't go near the "red" zone.
That's for very short efforts, though. If I pushed it beyond 10 seconds or so, I bet my HR would max out. Oh, well, I'm still working towards that century, so I need this.