I find it very hard to do anything after a full day, let alone give it my best in every aikido class. I do try to train to the fullest extent of what I have as often as I can; it is not my best all the time, nor can I do it every day.
Well, I think that is pretty much what we mean when saying "the best one can at any given time."
I certainly cannot do anything, from aikido to nursing to sewing, at the level of my very personal best moment/standard on every moment of every day, but better I show up and do the very best I can at that moment than sit at home and hope I am in my best possible shape another day!
I think one should train at the highest level one can at any given moment AND should be both looking to raise the bar and be encouraged to raise the bar.