Ten: here is my take on hydration.
Start well hydrated - seems obvious but not everyone does it. I will have a 500ml bottle of sports drink close to me in the few hours leading up to the race. I sip it in the car on the way up, the train wherever and stop abput 30 mins before the start - time to settle the bladder.
Then I will start with about 200 -250ml in hand - that way I can comfortably get past the scrum at the 1st water stop without disrupting my rythmn. By about 5 miles that will have gone....
Here is the thing - for me, it is more about the energy in the drink than the fluid if that makes sense. So I'm drinking more for the easily digestible carbs than the H2O. Water gives me nothing on a run.....OK, I know I need the hydration but I hydrate and fuel up at the same time.
For VLM there are water points at every mile from M3 and sports drink (Lucozade) at 5,10,15,19, and 23M. From memory these are smaller bottles (330ml) so you could pick up 1.6 litres on course and I seem to remember doing exactly that although I think I just ran with the last one and drank it after I finished.
These smaller bottles would have maybe 20g of carb of which 11g as sugar, so potential 100g in total. I think that is about 450 or so calories. If you are still with me on this, then I calculate that I need about 3,300 cals to get round and if I have correctly loaded my system I should start with about 2,700 cals on board.
Deduct 3,300 requirement from 2,700 = 600 calory deficit to be made up on route. Now I hope you can see how important the 400-450 cals that I can get for free from the sports drink is in the equation. A few gels 6+, evenly spaced out for me, some jelly beans & raisins etc I am on my way.....
I appreciate that I might have egg on my face "speaking" ahead of Ruth who is on here as an experienced professional, so I stand ready to be corrected, but the above got me safely around 2 maras last year without the dreaded "wall" and pretty much with negative splits in both. So unless I am way off beam, I'll be taking the same approach in the spring.
Final caveat: this supposes that we have moderate temperature no higher than low to mid teens C. Any higher and we would all need to adjust and yes, I would take on more water!