Is it sometimes OK to slow your pace during a marathon?

Conventional marathon wisdom says that a person should run the same pace or negative splits during a marathon.  While I'm sure that makes sense for competitive runners, I don't know that it makes sense for those of use who take 5-7 hours to complete the 26.2 miles.

Why?  The same marathon experts who recommend a steady pace or negative splits also say that we should run slower when it gets hot.  In fact, Jeff Galloway says our pace slows 20-30 seconds slower per 5°F.  If you do the math, that is in the ballpark of a FULL MINUTE pace slowdown per 10°F.

I happen to live in Florida and have done a few Summer runs when the heat index is in excess of 100°F.  I'm lucky to "run" at a 15:00 mile pace.  

Now, let's extend the math to an actual marathon for a 5-7 hour marathoner like myself.  Further, let's say the temperature increases from say 45°F to 75°F during that period (which has happened a few times during the Walt Disney World Marathon - most recently in 2012).

According to the Jeff Galloway correction, a person may end up running 2-3 minutes slower per mile if they are unlucky enough to be on the course when it hits 75°F.  By example, if you can run at an 11:00/mile pace at the start of the race when it is 45°F and thus are still out on the course when it hits 75°F, using the same level of effort your body might want to run at a 13:00/mile to 14:00/mile pace.

This seems to match my experience as a slow running doing several Florida marathons (at Disney).

If you are a slower running like me and are in decent shape, do you ever find yourself slowing down due to increasing temperature?  Or should we try to force our body to run at a steady pace or do negative splits (which is contradictory with the correction advice)?

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