Based on your previous posts, you haven't had the ideal preparation as you've made the (common) mistake of trying too hard.
Do your long run on Sunday, but if you don't feel right then don't feel that you must complete the whole distance. If you could find a 5.5 mile circuit this would help, as you could aim to complete four loops but you'd only be a couple of miles from base at any given time if it starts to go wrong.
I don't know what your target time is for the marathon, but I'd recommend you take a pessimistic look at it and adjust it to a slower target. Then you can complete the race and give yourself a rest. You can then consider learning from the training mistakes, and aiming to improve for your next marathon.