You have at most 4 weeks before a full marathon, and you have not run longer than 12 miles?
In a week or so, you should be tapering back your mileage -- but without more information -- it looks as though you have not run enough mileage to taper back from.
Also, it is getting to the point that a long run is not going to provide any physiological help.
You may want to consider not running the full marathon -- or work on some sort of run-walk plan to get through the 26.
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to get through the heat and humidity you may need to alter the time of day you run.