okay you real experienced gals, here's your chance to make some sense out of this stuff.
yesterday i rode 25 miles with this VERY experienced rider (10 miles was the race) and he's a spinner. I rode the same course with my husband a few weeks ago and the difference was very surprising to me.

when i did the race with Peter, my legs burned for 10 miles. I never really got to push though!

when i did the same 10 miles with DH I could feel my heart rate going quite high on the 12% grade. I really pushed up that hill. And my legs never burned, well maybe a little, but not the whole 10 miles..
I am certain that my heart rate never went up that high with Peter, because we were spinning more.

When we were spinning, it felt like my legs were wasting a lot of energy, but that's NOT what my heart said.
I plan to work on this on my wind trainer this weekend, even though I do not understand it!