I have sort of the same question. Have the C25K app. LOVE IT. Had planned on progressing from the 5k app. to the 10k app to the 13K in prep for a half marathon in the spring. I was just looking at the 10k app description and it basically said that someone who's finished the 5K app could just "skip ahead" to where that app left off......so basically that app starts at square one (where the 5K app starts) and takes someone to a 10K.......so basically I'm waisting my time with the 5K app and could have just downloaded the 10K app and been just fine.
That said, there is no such language in the description for the 13K (or even the marathon app)....so I don't know if those start from "scratch" or do they pick up where the others left off.
I don't mind paying the $10 for them.....but not going to spend the money now if I don't have to.