Find a way to make your contribution to your startup less about 'hours' and more about 'results.'
Make sure you're putting your time and energy into the highest value tasks, or the ones you truly enjoy the most. It will feel less like work.
Then, hire great people (pay them well) for the rest. Don't micromanage but set laser-focused goals and expectations, with checkpoints daily/weekly/monthly. Hold those people accountable. Fire fast if they don't keep up with the speed of your business.
Most importantly, don't start a company you don't love and don't try to solve a problem you're not truly passionate about.
It may sound a little 'mushy,' but I believe it's ok to use the word love in business. ;)