I have those days and I mountain bike regularly!
That's why I stopped a few years back. I was spending more time worrying about injuring myself than I was enjoying the trails. I still ride super-easy stuff like CCL and Bangert Island, but I think I'm done with anything technical. I'd just rather run it and, frankly, if it's that technical, I can do it almost as fast on foot.
Eric, even as slow as I am on foot that's often true (or close to true) for me too. Frustrating because I love mountain biking when it's fun.