My advice is similar to six-gun's. I recommend running MWF, if not MT/TF or even MTWTF.
To build up physical/cardiovascular endurance, as well as mental, I recommend running with a buddy who's at a similar level as you.
Find some place you can run for at least 7 miles without turning around. You should never actually have to run this far, but it's there just in case.
With your buddy, go on runs. Run at a pace where you can talk with him. Maybe not have a lengthy discussion, but where you can converse, and every now and then take a break for a few moments to just breathe while running. It shouldn't be as easy as talking while walking, but you should be able to converse.
Okay, now, at this pace, go running for time. Run out for 10 minutes, turn around, and run back. Don't stop at all, except if traffic on your route forces you to. Each week, add 5 minutes to your total run. So start by running a total of 20 minutes, 3+ times per week. Then 25 minutes. Then 30. Once you get to a point where you're running for 30+ minutes before turning around (60+ minutes of non-stop running, at a conversational pace), you're good to go and can either just maintain your times and do speed work, or continue increasing your times till you run out of distance, or whatever.
Running for time, instead of distance, greatly increases your ability to run those distances. Because you don't have the goal "I need to run x amount of miles, then I can puke and fall over." It's "I have to maintain my run for x amount of time, then I have to turn around and run home."
And for the testimonial, this method got my 2mi run time from about 14:30 to about a 12:30 in 3 months.