Part of me wants to side with Pete and say, race it. (Assuming you're not forced to sacrifice too much of your marathon training in the week or two following, though with your race more than two months out, that's barely an issue.)
But I think the answer also depends on what your personal weaknesses are. If it's sticking to a goal pace, then running at least the first 10k or even 10 miles at marathon pace might be helpful.
Pacing is definitely a weakness for me, but I also love to race, so I'd probably split the difference along those lines. (I did that once with a 10-mile race about a month and a half before a marathon, and it was a great workout. By holding myself back in the first half I was still able to finish decently, reeling dozens of people in. It was a huge confidence boost, and a lot of fun. Of course, in the actual marathon, I forgot everything I'd learned, went out too fast - and paid for it.)