I've been discussing my injury on running.about .com with Boscho, who is both a runner of many years and a medical doctor (http://forums.about.com/n/pfx/forum.aspx?nav=messages&webtag=ab-running&tid=4400), and he is helping me have a better understanding of why I was injured.
After I completed the Salt Lake City half-marathon in April 2007, I decided I wanted to have a faster LSD pace. My LSD pace at that time was about 11:30. I reduced my distance from 33 mpw to about 20 mpw and began to run a bit faster. No intervals, no fartleks, just running a bit faster. Then in October I decided that was enough of that, and I changed to increasing my distance back to 33 mpw. I was up to 7.5 miles (from 5 miles) when I was injured. Boscho has helped me realize that I should have not increased my distance for a while after leveling off in my pace, because my body probably needed more time to adjust to the higher stress of the faster pace. But, I felt good and unconsciously assumed my body was doing OK. After about 6 weeks of increasing my distance at the faster pace, my body gave up, and I was injured. I'm not sure how long I should have run the faster pace before I started to increase my pace, but I would guess at least two months. One thing to be learned from this is that higher stress may lead to injuries but those injuries may not occur for several months.
Another factor that is involved with this discussion is how often I should make increases in distance. I've been making weekly increases, but I'm wondering if, especially at my age, I should only make increases monthly. I've asked Boscho for his opinion on this. Also, I want to look in Galloway's book on running until you're 100 to see if he says anything about the frequency of increases.