Running and triathlon coach Brett Stewart gets this one from clients a lot: Between races that sell out months in advance and things that get in the way of training—like injuries, travel, and plain old busy schedules—it's not uncommon for athletes to show up on race day completely unprepared. But, depending on your background and overall fitness level, he says, there may be hope for you yet.
First, consider your past running ability and how it measures up to your current goal: "If you've never run farther than a 5K and you're signed up for a marathon, there's truthfully very little you can do to be ready in such a short time," says Stewart, author of 7 Weeks to a 10K and co-author of 7 Weeks to Triathlon. "But if you've raced this distance or close to it before and you've still got a solid base, you should be able to crash-course train for a few weeks and finish at least respectably.
In this case, "solid base" could be defined as the ability to run three miles without too much trouble. It also helps, of course, if you've been cross-training with other activities, especially ones that are heavy on cardiovascular fitness. (Weight-lifting: OK; jumping rope: much better.)
Ramping up your mileage too fast puts you at higher risk for injuries, says Stewart—plus, you'll be sore for days and your performance will suffer. The 10-percent-a-week rule is a good one to follow, he says, although if you're generally fit and pay close attention to how your body's responding, it's possible to go above and beyond when necessary.
So instead of jumping into a 16-week training plan at week 13, use the next few weeks wisely. Run every other day: three miles, then four, then five. Take two days off, then run eight miles—your longest run before your race—and then start to taper back down using the same schedule in reverse. Take the three days leading up to your race off completely, so you won't have tired legs for the main event.
"If you can successfully complete at least half the mileage before the race itself and you come into it injury-free and well rested, you should be able to finish," says Stewart. You should, however, set reasonable expectations for yourself: You probably won't PR, but your "Plan A" might be to aim for a time close to what you've done in the past. "Plan B" could be to finish without walking (or to just finish, period). If you're coming back from an injury, you might also have a "Plan C," to pull out entirely if you feel it flaring up.
Bottom line: If you're truly starting from scratch, it's probably smart to downgrade to a shorter race—or at the very least, approach this one with caution. But if you're in decent shape and can easily run a few miles (and if you start training today), you can most likely make it through without too much pain or embarrassment. Good luck!