For me, exercise is key. If I get in 30+min of exercise after work it's like hitting the reset button on my energy and I can squeeze out another couple hours of productivity.
On the days when I don't switch gears, there would be no way I could do focused work for more than an hour or two.
Honestly, if I ever really wanted to do a computer-based startup, I would leave my 9-5 job and do landscaping or something physical during the day. I don't think it's realistic to ask your body to be in front of a computer for 12 hours a day.
For the last 2 or 3 years working in agencies, you need a lot of extra willpower to get back on the computer after work.
I work best late at night and early in the morning so just try and fit my side projects in around that time, when I'm most productive.
You have to be disciplined though. On the days where I skip the run to get more coding done I invariably actually don't get more done as I have less energy.
Exercise is so so important.