The bride is coming round on Wednesday to talk it through with me, and I plan on going to the venue this week to check out the lighting and possible shot positions etc.
It all really hangs on this meeting, but please don't tell the bride you have contemplated backing out, unless you have a better option for them.
I've no idea what help you might need, because I have no idea what you know and can do, or what the B&G expect.
Read the recent 'first wedding' thread, it contains many of the likely bad outcomes. The biggest lesson is: Take control. You're not required to orchestrate the day, in fact most of the time people don't realise we're still there, but when it comes to the posed stuff, you need to be assertive and confident, take control, be funny and efficient.
If you're not, you could end up missing lots of the required shots. The 50mm is capable of shooting in most wedding venues, keep an eye on your shutter speeds, noisy sharp shots are keepers, clean blurred shots aren't.
What is missing from that other thread is the fun, those of us that do this regularly don't do it for the money, or because we enjoy the stress. We do it because it's a lovely feeling to be so close to people on the happiest day of their lives. It's fantastic to deliver them images that bring back those emotions.