The thing is in today's world of small apartments, noisy kids, and demanding jobs does not allow having the luxury of following a fixed schedules. I see blocks of time during the day getting wasted and I am not able to muster enough courage to claim those. On the other-hand is Cory Doctorow who is able to use these 20 minutes time blocks[1].
On the problem of not able to finish. I think Peter Elbow in his book Writing with Power[2] suggests many different strategies but they basically boils down to separating writing from editing. One technique he suggests is to divide the available time into 2 parts. Write uninhibited during the first time period and only do the editing in second half.
[1] http://www.locusmag.com/Features/2009/01/cory-doctorow-writi...
[2] https://global.oup.com/ushe/product/writing-with-power-97801...