I've determined to look past my age as the cause of this (too depressing), and I think the root cause may have been the first run we did up in Chicago a week and a half before. The snow had melted in most places, so we expected the 9.5 mile trail to be fairly clear and dry. However, because the previous snows had been packed down by skiers on the trail, at that point it was an icy, slushy mess. We ran anyway, and the first three miles were kind of a fun challenge for me, not having been up in any kind of snow or ice in years. (Don't ask me how I felt about the remaining 6.5 miles.)
There were a few slips and slides on the ice - one in particular had my feet sliding out to the side from under me and I twisted my body around pretty strongly to keep myself upright. I think that twist was the culprit. A few days later, I noticed some mild pain in my back - something I dismissed as being a part of "that time of the month." Then Wednesday happened.
Yes, the deeper root cause is probably still lack of core training, but I was glad to have another reason to stick on this. I suspect that I'm not the only runner out there neglecting my core training and I am not reading many posts from other runners finding themselves frozen on their hands and knees with their backs locked in waves of pain.
The same day I hurt my back another blogger in the Running Blog Family, and I'm embarrassed that I didn't record the name to give credit (if you read this, let me know and I'll credit you!), posted a suggestion that runners should make a new years resolution to focus on core training this year. Good suggestion. He also linked to a few videos on Runners World that showed Matt Tegenkamp's core workout. I'm afraid those workouts would not leave me with enough time to actually run, so I found a few others on the site that looked like I could fit into my weekly routine.
RunningDVDs.com looks like they have some good videos, plus they have a free downloadable PDF showing a couple of good strength training routines.
After running through my Christmas and New Years vacations like a champ, I haven't run since 8 miles last Monday. I'm feeling fat and unhappy. I'm going to give 3 miles a try today, and if they feel okay, I'm going to do 3 more. I'll have to revisit my training plan for the March 30 marathon, but I think I'm still good to make it. I intentionally made it a longer plan than usual for just this kind of reason.