I can relate also. I'm 48, and I can go hard for about 3o minutes, and then it seems like every four or five tricks I do after that I have to sit there for about a minute or so to re-energize, and then I can do another four or five, rinse and repeat. I don't compare my old days in the mid 80s where I would be out all day in the summer from 9am to like 11pm, and still not be tired, but I appreciate that I can ride for about two good hours before I have to call it a day of "practicing". I still ride for another hour or so just fooling around after that, so I'm quite happy.