Yes.
Well, maybe not - I suppose you could be one of those people who tolerates extreme pain very well!
You'll get round OK, but to get under 3:45 you'll really have to be able to keep performing after 3 hours, and that means training past 3 hours.
Long ones should be slow. Racing a half ought to kill you, as should racing any distance. If you had lots of miles left in you, you would have been slacking. To go long, you need to go slower, and there's really no advantage gained in getting quick times for the long run other than the confidence gained from having performed at somewhere near the level you'll need on marathon day.
So take it nice and easy - 9:00 pace, 9:30, maybe even 9:45, and just keep going through the pain. The ticker will be fine, the joints will hurt. It'll make the next one much easier.
Finally, a piece of advice that I think is extremely wise is not to get too hung up on a time for your first marathon. Play it conservatively and enjoy the day.