Disclaimer: I am not an expert all things strength training. These are my observations based on what I think I need to do to reach my goals.
Strength training is like the Garmin. It's one of those "why didn't I do this sooner?" things. I did virtually no strength training while I was preparing for my first and second marathons. My lack of strength training bit me hard in marathon #2. If I really think about it, it bit me pretty hard in marathon #1 too (although I was lucky and powered through the pain). This time around, I decided I cannot neglect it.
I am about 2 months into doing more strength training in my regular workouts. So far so good. My quads did not get bigger. My arms are more defined. I'm not spending a huge amount more time working out. I am faster. Well, at least I think so. I didn't have an accurate sense of pace until I got my Garmin in early June.
I try to do as much as possible with an exercise ball (to work my core) and/or my own body weight. I head to the gym to use weights, but I have a mat, exercise ball, and foam roller at home. I am planning on adding variations, such as adding weight, switching from a body bar to handweights, change positions, add or remove the exercise ball, etc.
This is what I aim to do over the course of a week:
upper body (bicep curls, tricep presses, and upright rows)
core (abs and lower back)
I don't always get to everything, but most weeks I do. I have a rough plan and then do what my body feels up to. For example, I skipped the pushups last week because my arms and chest were sore. Everything in moderation.