Not exactly uncommon, but usually starts earlier, like 5 to 8 days before.  You may find that it clears up by race day.  BUT . . . be careful after the marathon, when you will be more susceptible due to a slightly depressed immune system.  Treat yourself well the week after.

Len

Usually a generic cold will run its course in about a week. You're already doing a lot of the right things to prop up your immune system - you want to prevent a potentially more serious secondary infection. (Although be careful using zinc - some of the topically applied nasal gels and sprays can do permanent damage to your sense of smell - see for example  http://www.sciencedaily.com/releases/2009/06/090618130709.htm   ).

Also stay away from other people who are sick so that you don't pick up what they've got and compound the problem.

I agree with lenzlaw. After my last marathon I got a simple bug that just gave everyone else in the family a fever for a day (except the 6 year old that never even got sick), but put me down with a fever and sore throat for 3 days. Make sure you keep the immune system boosts (vit c,e,a , hydration, rest) up for a week or two after the marathon. Your body is going to recover from the marathon the same way it would as if you had surgery.