I've just had four weeks off running with a viral infection, pleurisy and likely whooping cough. I've was on steroids for a few days then switched to a steroid inhaler . My Gp said to return to running slowly and walking/ running. I tried on Saturday and managed a very slow 4 miles but had to stop and walk five times with cough. I have to admit I didn't ask Gp About marathon training although I had entered for Copenhagen At end of may 2013. I have run marathons before but was giving myself a break before starting in the new year but could run 15 miles for a long run anyway.
I would say proceed with care. Marathon training is hard work on the body and especially the long runs. Can you do your usual runs eg six to seven miles and how do you feel after that? I think you would need to try and do a couple of weeks where resuming normal running minus long run before starting them again. This is what I read on a website. When you do resume long runs, go slower (time on your feet is a good thing anyway) And make sure you don't end up leaving out shorter runs as well as doing long runs as your weekly mileage needs to remain reasonable for being able to get around the course. You might want to think about only doing one 20 Miler in your training rather than two. If you do resume long runs, have a nap within two hours if finishing after having post long run food. If you find you are struggling with long runs or overall exercise, you probably should consider derferring. Don't run a marathon when ill - a friend did this and was unwell with nothing specific for a year or more afterwards and this affected her running too.
My plan for myself is two more weeks trying to run short runs before considering a ten Miler and seeing how I feel. I had hoped to train for a sub four hour this time but i might need to reconsider and train for a finish or if not recovered in few weeks time, it will be a short break Abroad but without running the marathon.
Hope you feel better soon. Oddly two other people from my running club have come down with nasty infections including pleurisy over the last few weeks. Must be some nasty respiratory bugs around.