Dave says, “To get through the last stages of a long run you need to get your nutrition/hydration right and have trained well. Practise eating early in training and make it an essential part of your long runs. Find out what works for you. An accepted intake is about 60g carbs per hour so don’t wait for the last 6 miles before taking on fuel. It’s the same with hydration, sip little and often rather than stopping at stations and glugging loads. Try electrolyte drink to replace minerals.
As for training, it’s a simple matter of putting the miles in. If you’ve got a couple of years running under your belt you can probably run 6 days a week. Many people will build up to a long run of 20 miles about a month before the race. Include long runs with the last 40min at a faster pace to get used to running on tired legs. Getting fit is a gradual process, stick at it!”