I have been saying for a while that if you run, you need to strength train. As a disclaimer, I am not an expert on strength training. But I can tell you that strength training made a big difference in my running. I developed a strength training routine that met my needs, matched my goals, and fit in with my time constraints. This routine worked for me. Your mileage may vary.
You need strong legs for running. Contrary to what most people think, running does not make your legs stronger. It actually makes your legs weaker. You also need a strong core. This is especially important when you start to get tired and your form gets sloppy. Strength training, if you do it right, will not bulk you up or make you overly sore. It's like anything else. If running is your main focus, that is fine. Just don't forget your weights.
After having massive quad muscle failures at both RnR Mardi Gras and Cox Providence and being sore for days afterwards, I decided I could not neglect strength training any longer. I added in strength training even before I officially started training for Chicago and kept up my routine for Shamrock. It made a huge difference. Both Chicago and Shamrock went so much better, not just in avoiding massive quad muscle failures but also in having relatively quick recoveries. I ran a half marathon for fun the weekend after Chicago with no problems at all. I was back running at my regular pace a week after Shamrock. Really good stuff.
My marathon training schedule looked like this: Su - cross-train, M - short run, Tu - sorta long run, W - short run, Th - rest, F - sorta long run (sometimes at goal race pace), Sa - long run.
My strength training schedule looked like this, usually about 20-30 minutes at a time, and mostly with just my body weight, a mat, and an exercise ball:
yoga - one-hour power vinyasa flow class every week. My cross-training activity of choice. I'm going to have to find a new yoga class since I like it so much. Yoga is very complimentary to running. The class I went to felt like a combination of flexibility and strength training. Don't think yoga is strength training? See what happens if you learn how to do some of the arm balance poses. The first time I held crow, my upper arms were sore for 2 days.
Monday evening (or sometimes Sunday after yoga), always with the DVR
calf raises - 3 sets of 20 of 3 different variations (toes straight ahead, toes turned in, toes turned out) off the edge of a stair. Dip your heels below the stair, then rise up on your toes.
lunges - 3 sets of 20 on each leg with an exercise ball. Put your back foot on an exercise ball, then dip down and backwards. You need a mirror, at least at the beginning, to make sure your form is correct. You don't want your front knee to go over your toes. These were *really* hard at first. I still lose my balance sometimes.
core - 5-6 minutes of abs on an exercise ball, then 3 sets of 20-30 lower back exercises. For lower back exercises, I kneel with my stomach against an exercise ball, put my arms straight out in front of me, and lift up. You can start off with your hands on your lower back, then hands at your ears, then arms straight out in front. I usually try to make a triangle with my thumbs and index fingers to help keep my arms straight. You can also do these lying on a mat without an exercise ball.
Tu or W evening
planks - four 1-minute planks on my elbows and toes, sometimes fewer planks but for more time, sometimes side planks
pushups - 3 sets of 20 on my knees
Exercises with Weights (usually Tu or W morning when I went to the gym)
upper body - 3 sets of a rotation of bicep curls, tricep presses (hold weights behind you w/ palms facing up and press up), and upright rows (hold weights in front at your belly, and pull straight up to chest height). I do all 3 exercises and then have a rest and stretch. I started with an 18-lb body bar and then worked up to two 10-lb hand weights. Every once in a while, I switched to two 12-lb hand weights.
chest - 2 sets of 20 of regular and 1 set of 20 of wide (think goal post arms) chest presses with my shoulders on an exercise ball
squats - 3 sets of 20 holding two 12-kg kettlebells (a wee bit more than 50 lbs) with an exercise ball against a wall at my back
On days I ran and did strength training, I always ran first. It's debatable which one you should do first. Running was my priority, so I did that first. I tried to do my weights session on a day when I rested the next day, so that if I was sore I'd have some recovery time before I ran again. Strength training did not take up an inordinate amount of extra time above and beyond what I was already doing (plus I got faster during this time, so my total exercise time probably didn't change much). I incorporated variations every once in a while. I listened to my body if it was telling me to skip something or take it easy. I eased up the week before and after a race. In general, though, I didn't skip a lot of strength training. It was an important part of my training.