hi Jude,
You can certainly have some sympathy from me. I bruise like hell whenever I go caving, although I'm not doing as much in a month as you are in a week! I have very narrow blood vessels (have been barred from giving blood after numerous time consuming fruitless attempts, but useful at the hospital because they get the med students to practice on me as a tame awkward patient), so I've always assumed that it's because they pop easily being smaller and less tough. There may be something in the theory about experience lessening bruising, as I've found that I bruise in less places now than I used to. However, my knees down to my ankles is always black and blue on any trip that involves crawling ... and that's with knee length wetsocks, furry, meander suit & kneepads on.