Steve, great TR - congrats to you both!
Sounds like another day or two of acclimatization may have made the difference. Altitude sensitivity and conditioning are two completely unrelated things. Many a marathoner has flamed out on the switchbacks thinking conditioning will get it done. Even with the problems you experienced, though, you were faster than the average of 10 hours ascending.
Yes, a Whitney dayhike descent truly sucks. The switchbacks go on forever and the rock between Trail Camp and Mirror Lake just pounds your knees and quads mercilessly. And that 200-plus feet of ascent on the return at the JMT junction is none too welcome either.
Good call on Muir. When you're solo and it just doesn't feel right, discretion is always the better part of valor. Especially if "up" is tricky in a C3 or 4 situation - descending is always twice as difficult, and that's usually where bad things happen.
Congratulations again!
