Because I ran 12 miles today and I felt fine. My hip didn't hurt at all. My achilles tendon behaved itself and only hurt once, when I leaped too enthusiastically from the grass verge back onto the road after being forced onto the verge by yet another 4x4 who veered too close in an attempt to have a look at me. Even more amazingly, it's now five hours post-run and I only have minimal stiffness and soreness. I'm quite bemused by this and keep getting up and walking around to see if my hip has stiffened up yet. (Nope, not yet.)
I've been trying to figure out what I did differently with this run but the answer is, not much. I warmed up a bit more beforehand. I started off more slowly than I usually do and did a 4 min/1 min run-walk for the first five miles (except for when I forgot about it because I was enjoying watching the herons and the swans and the snow-covered hills across the water). I gradually increased the time spent running over the next four miles, and ran the last three. I drank some Lucozade as well as water. But that's about it.
The only thing that worried me was when, at about 2.5 miles, I felt an oddness in my left calf. I'm not sure what word describes it - a silent pop? a ping? a quick cramp? or something working itself loose? Whatever it was, it didn't hurt. I walked for a couple of minutes to see what was going to happen, but nothing did. And my calf did feel a lot more relaxed for most of the rest of the run. It's a bit stiff now but still doesn't hurt although I keep waiting for it to, resulting in an anticipatory limp as I move around the house and in Bassman unsympathetically rolling his eyes at me.
According to my GPS, my pace increased from a 12 min/mile at the beginning of the run to a 10 min/mile for the last three. And I could have kept going. Hurrah for me!