You seem to be in pretty decent shape. Maybe judge how you feel at the half and go for it. You can come through the half at 1:33 and still end up running sub 3 if you feel good.
Each marathon you run will be different. It's hard to predict how you'll feel. It's nothing like a 10k or a half. I Ran one a couple years ago where I fell at around 8 miles and banged up my knee. For the next mile, my knee was hurting pretty bad and, to top it off, I wasn't feeling all that great. I started considering dropping out cause I was limping so bad. By mile 10 my knee completely stopped hurting and I felt great the entire rest of the race.
As far as decent marathon times - anything under 3:20 is really good. Still, I'm always amazed by how so many people can run under 3:00.
Sounds like you've done all the work. Eat smart, and go out at a conservative pace. Judge how you feel at 6, half, and 20. If you need to, take a quick walking break every so often (it'll save you walking a lot later on). Don't worry about time lost - I've ran 2:45 off regular walking breaks.
On your 20 miler - if it's inside 3 weeks, I'd say cut it back to maybe a 13 or 15 miler at the most. The marathon is all about figuring out what works best for you and you probably won't figure it all out on your first one.
Good Luck.