I used to run all the time, I'm starting again in October to train for the Big Sur Marathon in April.
Building up mental endurance for me is the toughest part. I have had the most success with making finite mental goals as I go along to push myself a little more. For example, I fix on a light pole in the distance and tell myself "you will make it to that light pole." Once I'm there, I've already fixated on another object to push myself to. Keeping my mind busy helps me keep going and prevents the whole self-doubt thing from creeping in.
My other rule is unless it's a jog/walk day, I absolutely CANNOT walk during my course. It doesn't matter if I'm dying from a cramp or my jog is slower than a speed walk, not letting myself break from the jog helps me build mental and physical stamina.
To build speed, I vary my routine a lot and combine it with cross training. One day I will run 3 miles, and every quarter mile stop and do calisthenics (pushups, jumping jacks, mountain climbers, star jumpers, etc.) Another day I will run 6 miles; the next I'll run 3 but alternate between sprinting and walking. I try to do a hill day once a week, as well as a day on an uneven surface like gravel, dirt, or cross-country, and if there are any mountains near you, make an attempt to get up to high altitude to train every once in awhile. I used to drive to 6,800 above sea level once a week and run 3 miles cross-country and WOW that is hard when you are used to 3,000 ft or less.
For cross training, I use home DVD systems and/or swim. I despise the gym, I'll go for pilates or barre-sculpt but that's it. P90x and Insanity are great for cross training, I'm not in good shape so I've been doing P90 and Brazilian Butt Lift.
The very last thing is regulating breathing, I try to take deep breaths, hold it for 4 seconds, and then release long and slowly. This burns like no ones business the first week or so, but it helps build your cardiovascular endurance faster and control your heart-rate. I've seen a lot of friends struggle with this, so I figure I'd throw it in.
Good luck!