Thanks again for all the great replies. I have been trying to pay more attention to my breathing, primarily to stop "panting", and do more controlled deep breathing on the hills. The exponential rate increase at LT seems to be what I am experiencing. I read an article on climbing where the author states that a dull pain in the muscle is acceptable, and does not necessarily mean you are heading toward muscular failure. I think I have been shifting to an easier gear whenever I felt the dull ache, which shifted the load to my respiratory system and away from the leg muscles. It seems, that I have to get used to the dull ache of muscle fatigue, and stop trying so hard to avoid the sensation.