Working Out in My Seventies

I badly need a product that will make me indefatigable.
I have not the energy to blow, and less to bat a bubble.
It is just unfathomable that I ever walked
twelve miles through the jungle and barely even balked!
At the high point of my youth, adventures were my thing,
but lately I feel much deprived of energy and zing.
Swinging in my hammock is now enough for me.
It seems to mark the zenith of my energy!!!

Prompt words today are  zenith, unfathomable, indefatigable and product.