I recall a post from ExPhysRunner on RWOL about this topic.

 

If you take off at marathon pace with no warm up, you don't save glycogen.  In fact, you expend more than you would be the typical marathon warm up of jogging a mile and doing a few strides.  The reason is that marathon pace is a lot faster than a jog.  For me, my marathon pace is roughly 1:30 faster than my recovery jog pace and 45-60 seconds faster than my easy pace.  That's not jogging.  That's not easy.  During that really slow jog as you warm up, because it's just a warm up jog, you'll be burning mostly fat with just a bit of glycogen burning.  Those strides will burn glycogen almost exclusively but they are short.  What both do is prime your body.  Then, when you take off at marathon pace, you are burning a mix of fat and glycogen.  Conversely, if you do no warm up and suddenly take off at marathon pace, you will be burning glycogen almost exclusively until you have warmed up.  The net result is that this 100% glycogen burn at marathon pace will suck up more of your glycogen than if you had done a modest warm up.

 

Another mistake people make is taking in carbs too close to the gun.  You end up causing your insulin to spike but I'm damned if I can recall why this is bad.  But, the bottom line was that your last carb intake should be about 90 minutes before the start so that your insulin levels are at baseline when the race starts.

 

Here's a good article from Running Times

 

What the Elites do for Marathon Warmup

Actually the initial pulse of energy comes from your CP system and then rolls over to (predominantly) your aerobic energy system after about 5 minutes. You really can't force your body to burn more fat regardless of what brands and people try to tell you. Your body knows that one mole of respired oxygen creates about 120 calories trhough glycolysis versus 100 calories from fat. It's also not the distance that gets you but the intensity. (Hence why ultra runners can run for 50-100 miles without a 'wall'). So going out too fast or setting your goals too high (ie a BQ when you're nowhere near BQ shape) is the key to hitting the wall. If you want to see some great formulas for figuring out fueling metrics based on your own personal qualities then check out this study on the metabolic factors limiting marathon performance: http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000960

I'm late to the discussion, as usual, but thought I'd chime in.

First, to address the somewhat sidetracked discussion of a warmup, I'm all for doing as little as possible while still getting a little sweat going.  I like to be able to really stretch before a marathon, and I can't do that if I'm cold.  (And yes, I know the merits of stretching in general can become a whole discussion of its own, but for me, my body feels better)  Basically, my warmup turns into about a mile of jogging, plus some (2-3) accelerations to MP that I for about a minute/minute and a half, with the number and duration depending on how my body feels.

As for the pacing itself, I also use (try to use) the Running Times article as my guide:

First 3-5 - slower than goal pace

Through 20-22 - maintain goal pace

Final 8-10k - accelerating to the finish

I think the biggest thing is that the first mile can't be too fast.  It's so easy to get caught up in the race excitement and go out as much full minute faster than MP if you're not paying attention.  That kind of damage likely can't be undone.

Just because I'm really proud of them, I'll re-share the splits from my PR race in April.  My goal was to go sub-3 (6:52), but I also was hoping for a little faster.

Miles 1-5:  6:56, 6:45, 6:45, 6:47, 6:49 - so a touch faster than MP, but I kept the first mile reasonable

Miles 6-10:  6:52, 6:46, 6:54, 6:48, 6:51 - doing my best to settle in right on goal pace

Miles 11-15:  6:47, 6:50, 6:43, 6:41, 6:48 - starting to sense that I can maintain my pace, consider accelerating more but holding back

Miles 16-20:  6:43, 6:44, 6:35, 6:36, 6:37 - listening to my body and aware that I can start to push a little

Miles 21-26: 6:40, 6:39, 6:35, 6:35, 6:38, 6:35 - running my fastest over the last 10k

Essentially, those splits followed the plan I intended to execute.  However, I still adjusted based on how I was feeling and chose to begin a slight acceleration around mile 16, as opposed to mile 20/22.  One of the keys was that acceleration was slight.  Throughout the whole race there was never any real pace spike, and I think that was key because doing that can really take an energy toll.  That's an important thing to consider when starting out more slowly too.  Trying to jump from way under MP back to MP in one mile, in my opinion, will take more out of you than if you get back on pace gradually.

What about the warm up aerobics some races provide at the start?

Haha.  I think if you're doing that, you're there for the social aspect of the race.  There's definitely nothing wrong with that.  But, if you're there to really race the marathon, you should be focused on what you're attempting to do and be totally in control of preparing your own body.

I think it kinda falls under the 'nothing new on race day' category (even though we've decided some experiments are race day are valid).