Book in shorter races during your marathon training. If you're 8 weeks away from a marathon, get yourself signed up to a half marathon. Getting used to running in a large group of people can be overwhelming at first. But it will also push you harder so you may run faster.
Have a plan. If it's your first marathon, work out what you're doing pace wise on your long slow runs. You should be doing miles 60-90 seconds slower on these runs than the time per mile on race day. Work out your pace, train to it and know how fast you're going to run.
Don't go out too fast on the day. You might feel amazing at the start, pumped by everyone around you. But push it too quickly and you'll fade before 26.2 miles is out. You made a plan, go with it. If you're still feeling great at 18/19 miles, see if you can up your pace slightly. But remember you've still got more than a 10k to go!
Make sure that you're hydrating the day before the race. Don't leave it to the morning. Aim to have your pee almost clear when you go to bed. Plus, if you're really hydrated the night before, you won't need to drink as much in the morning and the last minute toilet dash won't be an issue for you.
Leave yourself plenty of time to get to the start. If you're rushing, you'll feel stressed which isn't great when you're trying to focus.
Be kind to yourself: face wipes, flip flops and a massive jumper in the bag you leave with friends or at the drop off will be a welcome end. Oh, and whack something sweet in that bag too that you can digest easily!