I've tried a few training plans: SmartCoach through the Runner's World website (you have to be a member to use it now, but signing up is free), the FIRST program (see here), and Hal Higdeon's training program (see his website here). Runner's World has a new feature where they will customize a training program for you (for a small fee) - I haven't tried this program, but it's worth mentioning.
I like the SmartCoach program the best because I have the most control over the variables. I plug in a recent race time, how long I want the schedule to last and how hard I want to train and SmartCoach spits out a list of runs that I post on my bulletin board at work. No thinking involved!
I run between 15-25 miles each week. These runs are spread out over 3-5 days. I really don't like running 5 days a week so I try to combine shorter runs when possible (I consider anything less than 3 miles to be a shorter run). SmartCoach divides up runs into four different types: easy, long, tempo, and speed.
So what do these words mean?
An easy run is just that - easy. You should run at a pace at which you can carry on a full conversation. No huffing and puffing or gasping for air. For a lot of people, this pace translates into about 1 minute per mile slower than their 10K pace (i.e. my most recent 10K was at a 10:30 pace so I do most of my easy runs at a 11:30 pace).
Your long run should be about the same pace as your easy run although slightly slower is okay.
Tempo runs aid your body in building endurance to keep a faster pace over a longer period of time. You should be able to hold your tempo pace for an hour although most tempo runs aren't that long. My tempo runs are typically between 3-5 miles with 1 mile warm up and 1 mile cool down (at any pace). My tempo speed is my 10K pace so about 10:30.
Speed workouts are short and fast with periods of rest built in. The idea is to get your body used to running at a pace faster than your race pace (though not by much!) A typical speed workout for me is 1 mile warm up (at any pace), followed by 4 repeats of 800 m (1/2 mile) at 4:25 (so run 1/2 mile in 4:25) with 400 m (1/4 mile) rests at any pace, and finally finish the workout with 1 cool down mile at any pace.
here). Easy runs should be at about 65-70% of your max heart rate. Tempo runs should be at about 80-85% of your max heart rate and speed runs should be at about 90-95% of your max heart rate. Here is an explanation of how to find your max heart rate.
I heart rate trained all winter (while following a SmartCoach program for maintenance) and was shocked at how disciplined my heart became in just a few months. I went from averaging 166 bpm during a 8 mile run to averaging 161 bpm at a pace 10 seconds per mile faster. It may not sound like a lot, but the idea is that as you train your heart, you can run at a faster pace using the same amount of exertion (so as a result can race faster at a higher level of exertion).
And while I do find it necessary to have a training schedule, if there's a day that I'm simply not in the mood to run, I don't. I ran one summer during our vacation to Maine. I got up at 6am and ran up to 9 miles because I was training for my first half marathon and was terrified of not being prepared. The only thing that ended up happening is that I resented running for a good few weeks after my vacation because it hadn't really been a vacation after all, had it? I've since taken as long as two weeks off of running (for personal reasons) and can very easily get back on the wagon. So don't feel bad about taking a day (or two) off - chances are, your body is trying to tell you something!
*Disclaimer (because I'm a legal loser)* - I am not a medical doctor or a personal trainer. I am posting my training schedule and accompanying explanations for informational purposes only. Do not begin an exercise plan without consulting your doctor. If you feel faint or dizzy while exercising, stop immediately.