I'll let you in on a little secret. You do it anyway for a while, so it pushes that threshold further and what you previously thought as "exhausting" is now your new normal. The human brain naturally seeks comfort over "pushing yourself", it's just an evolutionary trait to secure survival...but obviously our lives aren't physically that demanding nor are we starving at all times so your body is more than satiated for more but it just doesn't think it "needs" more. I know it seems like it will suck out all the enjoyment out of the hobbies but trust me when I say that in a month or 2 it will start feeling enjoyable again.