I'm envious of people who can throw something together during the week and then dance it in a comp that weekend. I need several weeks of good solid practice. For me, it absolutely has to be committed to muscle memory so that when I dance it in the comp, I can be thinking about musicality, presentation and floorcraft as needed. If I have to think too much about mechanics, it won't place well.
I don't usually "walk" routines as such. When I'm starting to learn a new routine, I will dance it through without the music, a bit at a time, just to get nailed down what goes where. Once it starts to come togehter in my mind, I can start doing it with music. There's a check at this point to make sure it basically fits -- it's not too long, it stays in sequence with a typical piece of music for that dance, etc. This is also where I want to think about long and short sides, what should be done where depending on how the judges and audience might be seated in the venues you expect to dance in. After that, it's practice, practice, practice. Run through and identify any trouble spots, smooth those over, run through it again, etc.
I used to try to write routines down, but I've found it easier these days to just tape a run-through without music, verbally annotating things as needed. The video I'm finding is actually very helpful, not only for remembering things, but for checking the presentation. Sometimes you run into sequences that might feel good to dance, but don't look so good to the observer.
I'm currently working on a show routine. This is the sixth week, and it's just now that I've gotten where there's a reasonably good probability that I can dance it all the way through without any errors. However, it's also about four minutes long, so it takes me more practice to get it together than a 90-second comp routine.