I have been watching people run since the 60's and can't remember ever seeing a smile on their faces. Some times with biking i do. At edging in on 50 i started doing hill sprints 100+yards for 10 rounds on a 45+ degree hill. 2X a week. Started doing it 3X a week and started slowing down as i wasn't recovering that quick. In my 60's now and i have not done 1000 yards of sprinting for a while and 2x a week about the most i do and can still recover along with everything else i do.