A Perplexing Question

Late in the afternoon I was asked by a younger colleague, a much younger colleague if truth be told, Where do you get your energy from?

I was entirely stumped for an answer, the closest I could get being something along the lines of not being aware I had any energy at all. But that didn't sound even slightly elegant, and I like to keep a conversation humming along.

I suppose my inability to frame any kind of coherently suitable reply is linked to the fact that whatever I've got that keeps me moving and occasionally grooving doesn't seem to me to be energy in any real sense. I can't think of a word in English that captures the stubborn keep-going-ness that keeps me going. Just haven't the energy to think of one.