I'm an old geezer pushing 74 and I do a lot of exercise most days, very often hiking 3 - 4 miles up and down steep hills up to 16% or so. Sometimes when I start out I feel lousy and short of breath just walking at moderate speed on the level, but I find if I persevere for two miles or so I generally feel "new life" and can charge up steep hills with little or no exertion. It's a great feeling when it occurs and I take it as an indication of being in good shape............but I'm puzzled as to the reason of the phenonymun. I'm wondering if it has anything to do with length of time necessary to get your body fully charged with oxygen. Or should I just leave it alone and be thankful? Anyone else have a similar experience and answers for it?