If you're just getting into distance running, then gradually work yourself into 25 miles a week for a little bit, see how that goes for you. If you start running too far too soon you can get shin splints pretty easily, and you're gonna want to avoid those for as long as possible. Start off runnin 2-3 miles a day, 3-4 times a week, then increase that to 6 days a week, then start extending your runs to 4-5 miles. give yourself at least 2-3 weeks in each part of that. Once you get up to 25 miles a week, you won't have to worry too much about physiological difficulties (IE breaking yourself) if you start upping the mileage quickly after that. Will just have to think about not ramping up miles too fast for yor fitness level.