That’s the marathon. I would do two things. Increase you long run to 26 miles, that puts to bed the fear and late race slow down. I would also do 6-10 mile race pace runs, that will make your goal pace much more comfortable and you’ll be confident that you can hang on. Finally, you just need to try to apply that training in an actual race. Do have to do the work first but then you must practice application. And as you discovered, the body wants to slow down, you must will it not to.