I know what you mean about hoarding energy. I do that too sometimes. I think that it is a survival strategy we have to prepare for the unknown coming next. But in a exercise class or a run we usually have a good idea of what physical demands are so I have to make an effort to overcome that instinct.
Hit a new PR
on front squats (barbell on the front of your shoulders under the chin-its a little harder that having the barbell behind your head because of the balance issue) last night - 100lbs 3 rep max!