OK, I'll take a try.

: First, how might one determine distance paddled? We are always curious as to
: how many miles we have traversed, especially after swamp or river trips
: where twists and turns render the GPS of little use.

Why don't you believe the GPS? This is exactly the sort of paddling that I find the GPS useful in telling me how far I have actually paddled rather than the distance I've put between my boat and some fixed point.

: Second, we wonder just what sorts of rapids a wooden boat can safely handle.
: We're not looking for any whitewater adrenaline thrills, but where would
: you draw the line? Class II, class III, classIV maybe? We have already
: done plenty of riffles and class I stuff, so we know that's okay.

Like so much in paddling and in life, that depends. What kind of paddler are you? What kind of class II, II, IV rapids? I would take a longish wood-based composite kayak down the Grand Canyon, but probably not down a tight, rocky class II. Are you the kind of paddler that can avoid rocks competently, or do you have a plastic boater's mentality? i.e. How sensitive are you and your boat to occasional, or regular, hard whacks? Is your boat manouverable enough to make the moves you need to make on a particular river? There's too much variation in paddlers, rivers and boats to give any kind of generic answer to your question.

: We're having so much fun taking our little Mill Creek 13s everywhere that we
: begin to wonder if we'll ever get around to varnishing them.

That's the key. I consider paddling to be a competition run by the Reg Lake rule: Whoever has the biggest smile wins. (Reg says he didn't originate that, but I heard if from him and can't remember who he points to as the creator.)