Do I attempt 20 again before the marathon?

I need some guidance please. As you will see from my blog, I have been training for the Rocket City Marathon, which is three weeks away on Saturday, December 11th. My training didn't start out too well, but things have been much better over the past few weeks until this past week, when I came down with a mild flu/ cold. I was scheduled to run my final 20 this past Saturday, but all I had in me was 15. I did get a 20 in a while back and completed 17 and 18 the first two weeks of November. My original plan, as I have done in all of my other marathons was to begin tapering this week, with long runs of 16 and then 12 the next two Saturdays until the race. My question is whether I should attempt 20 again this Saturday or just follow my plan as scheduled. One positive note for this week is that I am off from work, which means I can get a full night's sleep before I run. I am feeling better, although I am not completely over my cold. Any thoughts or comments would be greatly appreciated. Thanks!