I found that back when I had lots of free time, my style when doing projects was: I start where I left off, I get engrossed and code late into the night, repeat that one or two more days, get bored and "burned out" from the project, don't touch it for a week or so (sometimes I work on some other projects), repeat.
Of course, that's not really sustainable. But 30 minutes does not seem feasible for me.
It's not like you're going for a 30 minute jog where you just run without thinking. Don't you need to take time to "warm up" until the project you're doing starts being fun? And how does stopping just when you start enjoying yourself not kill your motivation? And don't you think about the project throughout your day job (instead of whatever problem you're supposed to be working on)?