I have noticed that when a part of an activity is internally resisted it magnifies tiredness exponentially. When doing manual labour but under someone else's timetable the constant stop-start is resented. Internal opposition builds. This squanders energy like nothing else. At the end of the day you are far tireder than doing the same effort but without resistance. if you can PLAY while doing it- even better. Play is frictionless activity- energy is conserved and you will not feel anything like as tired as you do when you use 'serious' energy.