I've been thinking.

In the wonderfully informative Runner's World there was a little box with the "Advice from Great Runners" and one bit was: (to paraphrase horribly) if you're feeling good do more than you planned.

Now, I think this advice sucks. If you're feeling good push your pace: go faster, not further! When I plan a run I almost always (90%) have a set goal in mind; generally a mileage. Because I try and stick to this aim like glue it prevents me from telling myself, "I'm feeling a bit crap today, lets cut it short and do more later in the week." It means that (unless real illness, not a sniffle, or injury prevent me) I manage to plan my training in any given week, and stick to it.
This repetition and commitment is one of the big changes that has taken place in my life. Apart from work this is the one thing I've really stuck to. (I'd probably manage to stick with surfing if you could just walk out the door and do it!)

I think I would overdo it on days when I felt great - and then be buggered for the rest of the week. I would also be tempted to quit on days when things weren't going so well. For me it's not flexible. As soon as I decide what the run is going to be (can be as late a decision as when I take the first bound) it must be that - or I feel like I've let myself down.

My plan is always vague but generally in a week I plan the following:
one gym session (weights only)
a good 5K
a 4-5 mile run
a longer run (anything from 10K - 20K)

Sometimes the 5k gets done in the gym, then the weights session happens.
Sometimes the 5K or the 4-5miler ends up being done on the elliptical machine.
Sometimes I manage t fit in a 30-45min yoga session. (Hah! Rarely)

The commitment is a big part of running for me. The beginning was so hard and so humiliating there's a fair degree of fear involved in keeping going. I'm so determined never to let myself get to that point again, I have to keep putting one foot in front of the other.