On marathon day you'll be running 26 miles, so your warmup won't be as lengthy as you're used to. The following advice should help you with the logistics:
Jog easily for a mile two at the most just before the start of the race or before you line up. Exert yourself just enough to break a sweat no more.
Stay warm. If the marathon you plan to run is very large, requiring that you line up well before the start of the race, wear an old T-shirt and sweatpants over your singlet and shorts. Then, simply discard your old clothes just before you start to run. Or you can wear a garbage bag to shield your body from the wind, then throw it to the side of the road just before the start.
Hydration is vital, and you should be thinking about it right up to the start of the race. Keep sipping water and/or energy drinks until you are called to the line. Of course, all this drinking is going to fill your bladder, which, coupled with pre-marathon nervousness, will send you searching for the portable toilets. That's fine. Just be sure to time your visits so that you don't have to sprint back to the start line at the last minute.
Stay loose in line. Stretch a little, jog easily in place, but resist jumping up and down, which can cause calf tightness.
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