I have a lot of routes mapped out too. If I get done and still have time left I do exactly as you mentioned, keep going. Sometimes it's just another 5 driveways, sometimes it's mostly around the block, and other times it's been a whole mile.
Also, if it makes you feel any better - I trained a lot like you want to train when I first arrived here a few years ago. It didn't work. It took a long...long time for these guys to get through to me, but once they did the gains I've experienced...never would have thought I could be where I am now. Be patient, disciplined, and listen to these guys and you will be successful.
EDIT: the down week is as Ned mentioned, shorter and easier runs. If you do 13, 15, and 18 miles the next three weeks and need a down week then three easy runs of 3-4 miles should suffice. Then get going again and ideally the next cycle has more miles, maybe even at a quicker rate.