just want to offer my thoughts into this.
after doing massive 600km, you body/legs are likely to be very exhausted. depending on whether you've manged to fuel correctly (fluids, carbs and protien) It's likely that your carbohydrate stores are depleted.
I would estimage a requirement of 600kcals per hour, and a mix of carbs/protein at 4:1 such as High5 4:1 supercarbs (its also an isotonic)
This will also provide a steady supply of carbs and protein and reduce muscle wastage and leave you feeling not so sore the next day and speed up your recovery.
glycogen levels are low and your nervous system is likely to be very taxed. It's at this point you are susceptible to colds/viruses so it's important to keep warm and eat a good mixed and varied diet, small portions of carbs, protien and vegetables every 2-3 hours, when I say small, i mean a hand full or so of each group, eat little and often.
This does two things, supplies your body/liver/muscles with a steady supply of fuel to restore it's reserves, which can typically take a few days after complete depletion, and provide your body with the energy it needs to fulfil your basal energy requirements.
In real terms, 600k/ 375 miles is at least twice as long as some of the longest classics races which pro's do (like Roubaix, Amstel Gold,etc) but it's important to get good recovery and fueling.
Joe Beer's pod casts have got some very good snippets for recovering, such as alternate warm/cold leg dipping to the bath/shower, this can alleviate muscle soreness/stiffening after the event and aid recovery.
Drinking cherry juice also has antioxidants which flush out the lactacte and help with recovery.
Weighing yourself before and after the event will give you an idea of how much fluid you have lost, typically 1kg per 1 litre, so you should have a good idea how much extra you need. Being fully hydrated before the event helps tool (your pee should be straw coloured)
Try daily stretching to alleviate any muscle soreness and try some low intesity static riding after 2-3 days. I know guys who've ridden the ironman who've felt 'flat' after 2-3 weeks recovery so it's important to listen to your body, most of all get plenty of rest.
