Don't wait until the start line to begin thinking about hydration, instead drink plenty in the week leading up to your race. It will do you no good trying to drink gallons during the race to catch up: you’ll end up with a bloated, sloshy stomach searching for the nearest portaloo.
Once the race starts, drink little and often, having a quick sip of something at every aid station is a good rule of thumb for most people.
Running 26 miles in the heat, means you are going to sweat a lot and that means losing important minerals like sodium. If it looks like it's going to be a hot race day, (especially if you have a high sweat rate) then help yourself by taking in extra sodium the day before the race.
This doesn't have to be complicated just get some extra sodium from good sources, sodium tablets (Salt Stick are really good), our Sea Salt Flapjacks and just adding a little extra sea salt to what you are already eating. Start adding the extra sodium to your diet 18-24 hours before you'll be lining up on the day.
With all the extra sodium on board, you'll naturally want to drink a little more before and during the race, keeping you nice and hydrated.
This one's simple, if it's 25 plus degrees outside don't have porridge and a cup of tea for breakfast. It's just going to unnecessarily raise your core temperature up and make you work even harder to stay cool later on. Instead drink some cold water or fruit juice and have a chilled breakfast like granola or bagels.
If your body is already warm from being outside, don't burn yourself out before your race, doing the same warm up you would in December. Instead do as little as needed before you get to the starting line, focus on activating your muscles and getting your head in the right place. Then let the race warm you up, by easing yourself into your race paces, this also has the added benefit of stopping you going out too fast in the beginning.
If the weather is particularly brutal you won't be looking to set any PBs but that doesn't mean you can't still push yourself and finish in a good time. Just don't go out too hard in the first half of the race and risk not finishing. For the first 13 miles consider adding 20 seconds or so to your per-mile pace and then if you feel fresh you can push harder in the second half.
Don't forget your Sun Cream, drink plenty of water and throw water over your neck and wrists as often as you can. Keeping these pulse points cool will help you regulate your core temperature and stop you from suffering too much.
Other than that have a great race and enjoy being out in sun!