Its all about energy management.
The days of , "I'm tired , but I just keep going" are done.
I treat my energy like a gas tank .. when its getting empty , I feel tired , the leg flops more , I start to yawn. Napping helps, taking regular vitamins, and a good diet helps. I find snacks , like fruit etc will carry me through.
I am still working daily , though its a desk job. I have learned to delegate and get people to come to me in order to keep the energy up.
Hope this helps.