~~I recently turned 49, have been training for 19 years--different arts/lately Aikido--and have been asking myself this very question this past year or two. The answers I finally arrvied at were:
1) No matter the mood of the dojo I'm in, I'll train as I deem appropiate and practical for wherever I'm at any given day.
2) I will train, because I love it, for as long as possible. Just as I will garden, backpack and live my life generally.
3) I train with intensity. Intensity doesn't have to always be physical. Much less, actually, as time goes by and timing, focus and positioning make more sense.
My 2 cents