At about 2000 total hours, and 70+ years, I don’t find 5 hours in a day obviously tiring – if the later hours are in a warm aircraft. 4 hours in the Jodel at this time of year leaves me borderline hypothermic. My flying is usually below the hill tops, with a lot of control work.
The US is much less tiring for refuelling – no effort or wait involved.
I originally did a 30 hour PPL on Jackeroos (Tiger Moth modified) and didn’t find completing the course from 27/7 to 21/8 at all tiring – at age 24 and with many glider winch launches and landings.
When the Jodel was C of A, and could be flown at night, I often flew after work. At the end of the month, I found many mistakes in my fly-fuel cost entries for these night flights. 
I don’t find flying in itself to be very tiring, even after 3.5 hours “naturally aspirated” at FL110; however, if I haven’t had enough sleep the night before (which, regretfully, happens way too often), I feel totally wasted once I have landed.
