You CAN do it! Just make sure when you're training you don't just keep upping the mileage, you need rest days in there and its always good to throw in a work out that isn't running to work other parts of the body too! And sprint workouts will help your speed!
If you've got access, go swimming every now and then... when I was marathon training I tried swimming as a work out for the first time and I almost drowned... Didn't realize how tough it was, but it improved my fitness levels so much, so fast.