I managed to suffer food poisioning on a recent trip to Poland; spent the entire night with it coming out both ends and my head spinning around like the Exorcist. It's taking me ages to recover; tried a run tonight and only managed 2.5miles before desperately feeling hungry. 2.5 weeks out from the marathon and I am in rag order... 26.2miles should be fun then!
Have you tried using rehydration sachets to get your salt balance back under control? Food poisoning like that will throw all your electrolytes/salts out of balance and is going to take a while to get back in order. Your kidneys will be doing there best to maintain balance but a little boost from your diet may well help speed things up.