Definitely don't run. Get better, book another marathon for later in the year and enjoy it. Running a marathon has never to my knowledge been prescribed as a cure for a chest infection. Probably for a good reason.
There's always going to be people faster than you but the enjoyment in running for many is the personal achievement, getting a time in your first race then seeing if you can improve. And you will. Make sure you aren't pushing too hard though as you'll injure yourself, train smart.