Guided by Carmichael's "The Ultimate Ride", I've been putting in lots of foundation, endurance, and tempo miles at a high cadence. Re-reading his description of tempo, though, I noticed that it calls for low cadence so I adjusted my routine and did one hour of tempo and one hour of endurance. They are roughly at the same heart rate, but when I kept my cadence low, I was unable to get my heart rate up to the target zone and my legs felt like they were being pushed to their limit. I mashed as hard as I could for an hour and then switched from tempo to endurance with a high (> 90) cadence. Almost immediately I was able to get my heart rate up to the target zone and my legs were much less stressed.
Last year, my first season back after a long absence, what limited me on long steep hills was my lack of aerobic capacity, i.e. I would run out of breath. This year what is limiting me on those same hills is lack of muscle, i.e. my legs just can't push any harder but I don't run out of breath.
I'd read that high cadence demands more aerobic capacity than low, but I'd never experienced it so directly as I did this afternoon. Obviously, I need to do a lot more tempo riding at low cadence so my legs will come up to the level of my aerobic system.