How fast must i walk to cover 30 miles within 8 hours?

i would like to join the royal marines. before i sign up i would need to train a lot. as one of the tasks that you will face is the 30miler that has to be completed within 8 hours. my terrain will be normal streets and footpaths for my first attempt at this just to give me a fair idea of how fast i must travel. is this a good idea? and also how fast must i travel?