you should not fee drained at the end of it, (If you are you are practicing wrong)
I know what you're saying Bill, and in most cases you are correct. However, the type of practicing I'm talking about is what I, myself, do to get better. I think I said it already...taking a song and playing it in different keys, playing left hand arpeggios to every chord the right hand plays, playing left hand scales instead of arpeggios, playing a song in different styles, and all kinds of creative stuff. Believe me. I'm drained at the end of a session.
I've lifted weights all my life and one of the things we learn is: if you're not tired at the end of a workout, and your muscles are not pumped, then you've just wasted two hours of your time!
But....one of the members in the Synthzone said to me a while back: I have to remember that "what works for me might not work for the next person!" I do always keep that in mind.