Right then.
I'm working on the assumption this is your first marathon so apologies if that's not the case.
First thing to say is that a marathon is mainly about willpower. Yes you need the fitness, of course you do, but more than anything you need the mental resolve and the sheer bloody-mindedness to push through. You don't really need that in a 10k or a half; not so much. Just when those distances start feeling tough, the race is pretty much over. A marathon is different and the training is different. It's about grinding it out when it feels hard and finding the strength you didn't know you had. But it's also about listening to your body when your body tells you to stop and not picking up injuries that stop you getting to the start line.
So right now, have a word with yourself. You can do this, if you want it enough. It's going to be uncomfortable at times, it's going to hurt, but the payoff is exhilarating.
Have a proper look at your "niggles" now. Are they niggles (normal and can be run through) or actual pain (need to stop to recover). Be honest with yourself. If the former, then pick yourself up. Today was a bad day but tomorrow will be a good day. You can get out there and put your 14 miles in the bank. Pretend today never happened. And if days like this happen again, tell yourself you'll just get to 10k before making a call - chances are you'll have bedded into the run by then and feel fine to carry on (also you'll be six miles from your start point so you won't have much choice!)
If you're actually injured though, take today as a de-load week. The basic test is to hop on it. If you can hop without pain, you can run. If it hurts to hop, then rest, ice, compression, elevation this week and pick up next week where you left off.
Brighton is a great race and the energy on the course will get you round. You can and will do this if you tell yourself you can, you accept it's meant to be hard, and you know you're tough enough. You've got this.