4 weeks out from the 2013 Boston Marathon, in the stores and over the interwebs, many folks are beginning to ask us the same question: Do I need to buy new shoes before my marathon?

Chances are good that if you're asking this question you're in need of new shoes anyway and are merely looking for a bit of validation, but there are a number of factors that play into why you need new shoes, what you should buy and when.
Older, dirtier shoes...

Why do I need new running shoes before my marathon?
Think of your race as a long road trip, say, from Boston to someplace warm and happy (read: far, far away from here). You've got your whole life packed into a number of bags, you've got the car gassed up and ready to go, and you've got your favorite sunglasses on. But you wouldn't dare hop in the car and drive 1,200 miles without checking your oil, your brakes, and your tires, right? Think of your running shoes as your tires.

Your running shoes are, with very few exceptions comprised of a midsole made of this wonderful little copolymer called Ethylene-vinyl acetate (EVA), compression molded and tooled in wildly interesting and aesthetically pleasing (if you're a sneaker nerd) shapes. EVA absorbs shock, resists torsion (for you pesky overpronators), and protects your feet from things like, well, the ground.

One thing EVA doesn't do well, though, is survive repeated compression. The more you beat on your shoes, the more they begin to resent the very one-sided relationship they have with you, and eventually they fail. This point of failure is usually somewhere between 300-500 miles (depending on construction, temperature, level of use, and stature) from when they leave the comfort of the shoebox for the first time, and during that period the ability of EVA to absorb shock and control pronation degrades. 

Simply put, as a shoe ages through wear, it does a worse job of protecting its wearer from the harshness of running. Replace your shoes every 300-500 miles and you'll thank yourself for your prudence.

Ok, if my shoes are dead, then what should I buy before the race?
This one is easy: buy exactly the same shoes you've been wearing. You've spent the better part of the last three months using one, maybe two different shoe models to help you train and prepare for your marathon, and your body has become accustomed to a certain fit and feel. Why on earth would you ask your body, during your taper,to learn a new trick?

Since running shoe companies, like car manufacturers, change their models on an annual-or-so basis, you should have little difficulty replacing your existing model with an identical pair. Any running specialty retailer will do its best to carry inventory in key models and sizes throughout the year, so getting another pair can be as simple as filling out a form.

IF, and only if you find yourself in the unfortunate position of having a rare, hard to find, or discontinued model on your foot, should you consider switching brands or models. Again, your running specialty friends can help you find comparable models that will serve the same purpose as your existing shoe, so all hope is not lost.

Fine, I'll stick with what I'm currently wearing. When should I buy them?
Remember when shoes were made of things like leather, with metal lace grommets and natural rubber soles? Me either.

But those are the kinds of shoes that you really had to "break in" before you could wear them. Your average running shoe, made of polyester, nylon, and EVA is almost entirely synthetic, and many of its constituent components are pliable and flexible out of the box. 

The only break in that you need to worry about, then, is your body's break in period to a new pair of shoes. Understandably, a pair of shoes with 500 miles on them will feel decidedly different than the same model in brand new condition. Try one of your old shoes side by side with a new one and you'll know what I mean. That difference in feel can sometimes take a few miles to adjust to, so we suggest giving yourself about 30 or so miles of wear to ensure you are used to the shoes before your big race.

One last consideration: unfortunately the manufacturing tolerances in the various factories in which running shoes are made can lead to variation in stitching and assembly, and every once in a great while we find a dud

While quality control at the major footwear vendors is very high and the percentage of defective product we see is extremely low, the last thing you want to do is buy a pair of shoes the day before your race and just trust they're going to do the trick.

Running 26.2 miles with a blister and a calf cramp can be a really, really annoying (and painful) experience. Give yourself time and due diligence, and your feet will thank you.

Old, dirty, melted shoes courtesy of Clint Randall and Harvard Stadium.