Taxi.Idge,
As you probably guessed from my previous post I have only ever done the Rockport Walk, mostly as my silent protest that an 18 girl gets longer than me to run a mile and a half.
The Navy have been doing the RW for as long as over 40s have been tested (2003ish I think), so if you want good advice from PTIs who know how to structure a fitness package to pass the RPW, then they are your guys.
The PTIs at Culdrose always gave me good advice for my division when they failed, but I'm sure the, "I'm old enough to be your dad and I passed" also helped to embarrass them fit.
Cheers