Pull yourself together and stop this self pity. 
No reason why you can't go out and run a sub 3:30 marathon if you've done the training. 4hrs is a massive difference and probably not credible. OK - so don't put the pressure on yourself, but you'll still run closer to 3:30 than 4.
You can still do long runs with the runs, I should know, just carry some wetones with you. But if you have a chest infection still then stay home till your over it.
I can understand your doubts as I feel them for myself and was worse on my first marathon. I had 5 weeks off running and only did 2 long runs prior to the marathon. Just take it one day at a time. Don't do anything silly. You will enjoy yourself. The buzz you get when you finish is great. 