You have jumped in at the deep end, trying to comensate for time missed as well as probably still have the tail end of a chest infection.
Don't worry about yesterday - put it down to experience. Going from 14 to 18 miles is one hell of a jump esp after running an 11 mile run, 2 weeks off and a chest infection. Don't worry about walking home - it's all time on your feet / mileage, it will help with the mental attitude while running your marathon as you still had physical work to do past the point of comfort.
I would advice that you don't try and keep up with your friends unless you really need the motivation.
rest this week with 2 or 3 easy short runs. at the weekend go for your long slow run, if you choose to run with your friends only stay with them if it feels easy. The idea is just to run very slowly for as long as you can while still feeling comfortable. You may find that you need to run slower than you have in the past.
Slow and easy.
You can't run to your plan now, it really isn't surprising that the wheels came off. You need to reassess.
I think with an easy week this week you will surprise yourself with your next LSR - don't try to overdo things.
Good luck with the rest of your training.
I can only tell you what I would do which would be to run slow and see how far I can run then add to this next week and the week after with a much reduced tapper period and take the marathon very easy.
Edited: 05/09/2011 at 10:46