Marathon in 3 weeks and you are sick and pathetic. WYD?

I recommend not being such a pvssy

Its part of the game. Get used to it. The marathon is a (or should be) a long drawn out process. During that 6 to 12 months (and for some - a year or two) process allows for plenty of time to take on a 'show stopper'. You need to know when your body says NO and just deal with it. I got pneumonia 3 weeks out from a Boston Marathon and just adjusted my plans to grab the next one. Ran one Boston 4 weeks after a COVID booster shot and ran that one feeling like I had pneumonia with a 6:30 time. I developed a show-stopping Athletic Pubalgia (aka Sports Hernia) during my Boston qualifier one year - without time for a backup. You can cry all day long and throw junk around the house in anger - or you can just regroup, replan and deal with it.

The timing is great. You'd be starting to taper now anyway. Whatever fitness you were going to build is built. The sessions you were planning were probably overreaching, and your illness has come along at just the right time to stop you from doing that.

And with 3 weeks to go, you have plenty of time for the illness to run its course. Take whatever days off you need when you're feeling bad or feverish, or do some short easy jogging when you're starting to feel better. But don't try to sneak in workouts. When you're all the way recovered, you'll still have time for a couple of easy MP sessions to get a feel for the pace again.