I once asked asked my professor for advice. I said, I am drawn to both doing a startup and doing a PhD. How do I do both? He advised me to choose one. “Either one will take the whole of your youth and all its energy”, he said.

Then again don't forget being ambitious enough to be an ambitious founder, ambitious dad and an ambitious husband/wife/partner. And then being healthy enough to have energy to do all three. I keep thinking if I divide 24 hrs by 4, that would be the optimal time allocation to all 4 endeavors, but that's naive.