Sure you can't completely rewrite the billing system from scratch in an hour, but you CAN stub out the new architecture. Then each hour after that you fill in a method.
I feel needing 4-9 hour blocks of time means you AREN'T focusing, not that you are "IN THE ZONE"
This process typically only takes a few hours at most and usually illuminates something really unexpected about the problem space. Selecting for coherence is effective at reducing problem scope since it cuts through unquestioned practices: in a more coherent solution all the answers feed into each other.
In my previous methods I would just sit and stare at the problem and sit and stare at techniques until a match of technique to problem came to mind. That's why it took so long - there wasn't a philosophy there, so my thinking could drift in an undirected way for long periods and even get deep into writing code without doing anything effective.