I have done two marathons in two days because of the 50 state quest.
Although a slower runner than you, I would suggest that you add about 30-45 minutes to your first marathon, and walk breaks in between. If hills, walk the hills.
After the run, get a good nap in, then some food of some dry pasta and chicken.
You will be sore the day after, but just take it one mile at a time. Follow the same as before (an added 30-45 minutes to marathon usual time) with walk breaks.
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