I think doctors frequently say "10 days" because it's a nice even number and I think they figure if you go at least 2/3 of it you're doing well. That being said, pay attention to how you feel and remember just because you're not coughing now doesn't mean you won't cough a lot when you add the stress of running. And being as sick as you are will leave you easily fatigued even after you generally are "better." Sorry to sound like a bummer, just trying to get the full picture here. But... if you are feeling better, try a few easy runs and maybe an 18 a couple weeks before. You might only need a two-week taper anyway. If you feel good, go for it. If you don't, go the training run option and run with your cousin, and find another marathon coming up. Either way, I think you can run this race, just need to determine the effort you can expend. But I wouldn't do the 20 this weekend... I'm thinking try 18 (or 20) next weekend if you feel up to it. Then see how it goes.