Sure it works.  The Galloway method has allowed thousands (tens, hundreds?) of runners to finish a marathon.  Typically, though, Galloway recommends that you start your training with run/walk.  If you are up to a 19 miler at this point, why change your training program?  I would suggest taking a more flexible approach.  If you are concerned about stamina and know your race course, you could plan walk breaks at water stops.  That would be more natural at this point than trying to set up timers for a 4/1 run/walk pace, and would give you "permitted" walking breaks.

I used a run-walk strategy for my second of back-to-back marathon weekends last year. It worked out pretty well; I finished in a better time than I expected. The main thing if you're going to do this on race day is to make sure you're not right in front of anybody when you slow to a walk. I always checked over both shoulders and moved to the side of the road before shifting into low gear. Worked well, and I know the runners behind me appreciated it.

A useful run/walk calculator, even if you're not running ultras: http://runners.groups.vox.com/library/post/6a00e398c5c9dd0005011015fefa62860b.html

Don