There's a well known myth in running circles that if you want to run a marathon, you should run at least 20 miles in training. I prefer my athletes to run no longer than 2.5 hours - no matter what distance they cover. Running longer than 2.5 hours is very likely to leave you feeling fatigue and therefore impact your training over the subsequent days. There is simply much more benefit to having consistency in your training, rather big sessions followed by a couple of days of inactivity to recover. Its also an easy way to pick up injuries. A great alternative to a single long run would be to do say 2 x 1.5 hour runs within 12 hours as you'll get the combined benefit from both session. I'd suggest doing one in the evening followed by the 2nd run in the morning. This is less likely to cause injury but a great way to develop your aerobic engine and build up resitance to injury.