Please help me adjust my marathon training plan.

One, don't worry about a time goal.  Just focus on finishing as best you can.

Two, one run won't make or break you.  If you don't get that long run done, in the grand scheme it probably won't make a huge difference.

Three, run what you can.  Run as much as you can.  Plans are guides, not religious tomes.  Life happens.  Do what you can, and be honest with yourself come race day regarding your fitness.

You're making me nervous. My plan has me maxing out at a 20 mile run leading up to my marathon in March. I'm aiming in the 5:15 range, but there is a 6 hr cutoff for my race. If I were you I would work back up close to marathon distance slow(ish)ly.

x2, my plan also has my longest run at 20 miles.

To those worried about the length of your long runs:

There is no magic number.  20 is just a nice round distance.  If you train by kilometers, most plans put it at something like 30-35 km.  There are actually a number of people who don't run 20 milers, including elite level marathoners (the Hanson brothers, in fact, have training plans that don't go longer than about 15 or 16 miles).

Really, the way I've seen it, the higher your total weekly mileage, the less important the distance of the long run.  Of course, the converse is true as well; if you have lower weekly mileage, those long runs become much more important.

Then my long runs are critical!