I’m no professional but the only full marathon I ran about six years ago I managed with energy gels every 5-7 kms, some NaCl and whatever sports drink and obviously plenty of water there was. I did eat extra carbs (porridge, pasta) and drink more water than usual for a couple of days but it wasn’t very methodical. The day before the marathon didn’t eat/drink anything extra anymore.
The last 10 km was a pain and I probably was slightly low on energy but I did manage the marathon under 4 hours which was my goal. A really nice experience overall. 
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