Standard practice for runners heading into cold weather is to bundle up in wool or synthetic wicking layers, dressing as if the temperature were about 20 degrees warmer than it is -- that way you'll feel comfortable once your body is all warmed up during the run. But performing at a high level in cold weather, for more than 26 miles, introduces a few additional concerns.
Skip the ultra-breathable mesh running shoes and go for waterproof/breathable uppers. The waterproof/breathable membrane acts as lightweight insulation and keeps water out, but still allows your perspiration to travel out of the shoe. Pair those with wicking socks made of wool or synthetic materials.
Avoid cotton clothing, which will soak up sweat, chafe and chill you once wet. Dress in a close-to-the-skin wool or synthetic base layer instead, topped with another layer if you need it. Dress as if the temperature were about 20 degrees warmer than it is; you may be a little chilled while standing around but you'll be comfortable once you're moving and, most important, you won't end up soaked with sweat once the race is under way.
Ventilation zips at the armpits, full-zip layers you can zip open, or layers that are at least easy on/easy off mean you'll lose as little time as possible making clothing adjustments throughout the race. Once you start moving to warm up, keep moving; if you stop, you'll get chilled. If you can, wear an extra keep-warm layer that you can throw away or pass off to someone just before the race starts. Moderating your warm-up so your muscles get warm, but you sweat as little as possible, also helps.
Even if you're not a fan of wearing tights on race day, the extra coverage helps keep your muscles warmer, increasing your performance and reducing your risk of injury. That's worth putting up with full-leg coverage on a cold day.
Vests will keep your core warm while still offering good ventilation; in truly cold temperatures you can add a highly breathable jacket and pants, plus light gloves and a hat. Men may wish to add wind briefs, because external genitals need a little help keeping warm in low temperatures. Finally, don't forget a reflective vest, reflective wear or headlamp to make yourself more visible to passing traffic and other marathoners.
Apply BodyGlide, BlisterShield, SportShield or another anti-chafe product before your marathon, and carry a small tube with you to reapply as needed. You can also rub these products or petroleum jelly on your face as protection from any cold winds that suddenly kick up, or on exposed arms and legs to help keep them warm if you didn't dress adequately for the weather. Once you hit zero degrees -- or sooner if you're feeling uncomfortable -- wear a facemask to both protect your face and help warm air before it hits your lungs.