Running is primarily about raising your body's ability to tolerate heavy stress. The easiest, most efficient way (arguably) is to run a lot!.....or in the case of the struggling runner, RUN MORE. The more you run, the more time your body is stimulated to improve its "endurance" and the more time your body builds a resistance to stress.
The easist way to run more is to A) SLOW DOWN and B) RUN MORE OFTEN. You don't slow down for the sake of slowing down or run more often for the sake of running more often. You do it so that you can run more. If you struggle with running and typically get 2 runs of 45min-1:30 at 85% of max heart rate (Gordo's Moderately Hard zone), instead try running 6 times at 65%-80% of max heart rate ("steady" or even "easy"). Each run can be as short as 15 minutes. Once you get used to 6 short easy runs a week, gradually build the distance of 2 or 3 of them. Start off running no more pre week than you currently do, and then add 15 minutes a week over the course of several months until you get to 3, 4, ....as much as 5 hours a week of running (admittedly this may not be a "well balanced program".....but if you struggle on the run, you may need to shift your training to be "run heavy").
For the Ney Sayers out there, correct, this is not the end all be all of run training. Intensity is needed to achieve peak fitness. Time limitations can become an issue. Many have succeeded on a "hard all the time approach." However this is targeted at the struggling runner who has never tried a high mileage, high freduency, low effort approach. 9 times out of 10 I find this to be a neccessary 1st step in building a foundation for one's career as a runner. I highly recommend giving it a try for a good 4 month block and see the results.
http://forum.slowtwitch.com/gforum.cgi?post=1311512
