" Basically I just don't get tired until I'm getting both feet off the ground at the same time"
being of similar construction, what i do is to (serious UL'ers do not read further) is to go up to Auburn and load my pack with several choice rocks. then i walk, slowly to be sure, but breathing hard all the way, up to this EXtrememly tall bridge they have there (750' of gain), and then out to the middle where the suicide phone is...
from there it's an easy trick to drop the rocks back down into the american river, where they make a rewarding KA-POW like a shotgun blast. then i drop back down to the car and have a glass of wine.
this has proven, along with toting similar loads of rocks 1/2 way up mt diablo to eagle peak, to be the fastest, least destructive, most rewarding manner of staying in respectable condition.
stupid people keep throwing my cairn over the edge of eagle peak, but i have enough stones up there now to make a pretty decent one if i fetch them out of the bushes and stack 'em high.
the key pieces of this method are UP, and HEAVY. up is always good, and heavy is an acquired taste involving an old mchale pack padded to reduce the pain the rocks cause.
i read somewhere that some adventure racers are into similar dogma.