Welcome to the Forums! And congrats on running the London Marathon.
As far as nutrition goes, I'm a Disney Vacation Club (DVC) member, so I book a one bedroom villa that has a full kitchen for race weekends, so I can cook my own food. I live in Florida a 4 hours drive away. I pack a cooler with food for dinner my first night at WDW, some frozen items (soups, spaghetti sauce) and breakfast stuff - eggs, milk, oj, etc. I arrive the day before the Expo starts. After the Expo, I drive over to a nearby grocery store and shop for stuff for both lunches and dinners. Quality proteins like fish, chicken and lean meat. Vegetables, fruit, rice, bread, bagels are also on my list. I normally eat whole grain pasta and bread, but switch to the white stuff during race weekend as it's easier to digest. My dinners are what I ate as a kid growing up and still eat - a protein, starch (potatoes or rice), vegetable and salad. For races, though, I up the carb portion. I like to make a pasta that includes vegetables and chicken or fish. Lunches are usually a sandwich with meat and cheese and fruit or raw vegetables. And I eat yogurt every day. My pre-race meal is a toasted bagel spread with peanut butter and a cup of green tea (don't drink coffee). Filling, easy to digest and puts more glucose in my muscles. I'll bring a banana with me to eat while I'm waiting, as you'll have at least an hour's wait time before you start.
runDisney hands out a food box after the races, along with a banana. Frankly, what's in that box is mostly junk - cookies, pack of fruit bits, applesauce, tortilla chips and liquid cheese to dip them in.
I eat the applesauce and fruit bits, save the cookies for later and toss the rest of the box. When I get back to my room, I drink a big glass of chocolate milk, soak my feet for a bit, clean up and have a big breakfast with oatmeal and eggs to start that important recovery period. Once I've finished all my races, I then indulge in Disney food and adult beverages until I leave to drive home.
All of the on property hotels have what Disney calls quick service food courts where you can pick up pre-made items like sandwiches, salads and bakery items. Or you can order something. The Moderate and Deluxe resorts also have sit down, table service restaurants, in addition to the quick service spot. All rooms have a small under counter fridge in them that can store a limited amount of fresh food. All resorts, except for a few of the Value resorts, have coffee makers. All of the parks and Disney Springs have many options for food. I think it will be easy to eat on property and maintain a pre-race diet, just stay away from the stuff experts advise you not eat before a race - spicey foods, beans, anything than can give you tummy troubles.
As far as weather, temperatures for Marathon weekend are usually cool - 50s overnite and at start time. This year's race weather was unusual. Extremely cold in January for the Marathon, very warm - even hot - for the Princess Half in February and typical for the Star Wars Half in April. Lots of runners pack a cheap sweatshirt or hoodie to wear in order to stay warm while standing around in the corrals waiting to start. And then toss it along the side of the course once they've warmed up. runDisney then picks up the discarded clothing, cleans it and donates it to local charities. I remember reading after the conclusion of Marathon weekend that they were able to donate thousands of pounds of clothing to local charities. And there always the black plastic trash bag trick for staying warm.
I'm going to be participating in my first Marathon weekend next year, now that the Anaheim races are on hold. But just the 10K, as I've got all the Princess races the following month. I have no desire to run a full marathon, which my feet are very happy about.
Hopefully, we'll see you next January and at other runDisney events in the future. And you're not far from the Disneyland Paris Half.