I was on track with marathon training until July 4th. I ran the Sauvie's Island half, since then I have been running 20-25 miles per week but not doing any runs longer than 7.5 miles.

Is it possible to pick up the 21 miler and still do the marathon or is that a really dumb idea?

Its possible, and you might be able to do it, but in my opinion, its a really dumb idea. Why not be better prepared, finish strong, and feel good following a different marathon?
Good luck either way, I've fallen off in my training many times and can relate to your quandry.