I did a 10 hour mtb event on Saturday in the scotching heat- ended up riding for 11 hours in total with about 2x 20-25 min breaks. Tried to ride steadily and within my heart rate zone most of the time (max of 165) and tried not to push it too high. Hydration felt fine (I was using Torq grapefruit and drank about 5 litres- probably not enough given it was so hot) but I didn't eat very much at all either during the race or immediately afterwards.

Anyway, had a couple of days off the bike rehydrating and eating lots of healthy-ish food. I went for a short easy spin on the road last night and my legs were still tired and couldn't get my heart rate as high as normal.

How long should it take to recover from major exertion and is there anything you can do to help the process? I have another one in 3 weeks time and want to be ready for it and recover better afterwards.

Try taking the Torq recovery drink at the end of your event as well, particularly if you cannot stomach food. I would struggle to eat after that kind of effort in the heat. If you still have low appetite a couple of hours later then have more recovery drink. I normally find that having some recovery drink straight away gives my body some calories to be going on with then about an hour after I stop my appetite kicks in with a vengeance, if I don't get some calories in quickly my appetite can take ages to return which is really not good.

Not putting sufficient calories in after your event will probably hinder your recovery a bit however if I'd just exerted myself as you have I would be expecting to take a very, very easy week not just a couple of days.

I drinky chocolate milkshakes, find that really helps. After the last merida when I was out for 8hours I did half an hour on the turbo trainer the next day, think that really helped.

Trio thats just an excuse to drink choccy milkshakes, but yes they do taste nice !!

I do find it can take me several days to recover if I've been working hard. I find if I allow myself time to recover 3-4 days sometimes then I don't lose my fitness and I'm fine the next week.

Sounds like you need to get the calories in as well - healthy ish food is great but if you meant fruit/salad considering you have mtb for approx 10 hts you have burned a few thousand calories then the fruit/salad wouldn't replace the calories.
Also the first 20 minutes after any exercise is your gylcogen windown where your muscles will make glycogen the quickest - so if you can't stomach solid carbohydrate (think ginger nuts biscuits and fig rolls!) then a carb drink would work as well as Julbags says.
Getting enough sleep also helps recover quickly, as does a massage (or second choice is the jacuzzi).