Just sticking my nose in here. I'm another friend of Mark & Bill. I've actually known they were in season 13 since early this year, because I was one of Bill's housemates at the time, but I had to sign a non-disclosure agreement, so I couldn't talk about it. Of course, I still don't know how they did, and what guesses I have come largely from what I've seen on the spoiler forums here, so I can't really add much.
I do think the LA Times guy got things about right regarding them, however. They're smart guys, and I think will not be prone to make elementary mistakes (like not reading the whole clue, something we've seen teams to). Bill is particularly strong on game theory. Mark is very well traveled, having spent time in Asia, Europe, and the Pacific; Bill a bit less so, though he's been to Europe a few times, and studied a little Russian in college, so he can at least read the Cyrillic alphabet with no problems. They're both very outgoing and personable, and will I think be very good at interacting with locals and getting favors from them. They also are both pretty easy-going guys who don't get seriously angry very easily -- I've known Bill pretty well for about 25 years, and I think I've only seen him get really angry about twice -- and I doubt they will waste much energy on serious bickering (non-serious bickering, OTOH, will be one of their major forms of recreation). Mark has done all manner of adventurous things, such as skydiving, scuba diving, volcano climbing, flying in a zero-g airplane and, as tcost pointed out, running with the bulls in Spain; Bill hasn't done so many, but there isn't too much he's seriously afraid of, and I doubt either of them will wind up paralyzed by fear. I think they can handle any food challenges better than most people, too.
Downsides? Well, as the LA Times piece said, there's the sheer physical side: both of them in their early 40's and neither is much of an athlete; though they did some training to get in a bit better shape for the race, they will probably have some trouble keeping up with the younger and more athletic teams. (Mark also has a terrible sense of direction, but he knows this, and I think part of their strategy is that he will always defer to Bill when there's any question in that area.)
I'm glad to see that they seem to be getting some popularity/ They're both great guys, though not quite the demi-gods that tcost makes them out to be -- but as I'm sure tcost will be the first to confirm, you can't really trust anything he says.
Oh, and while I have never seen Mark in pink spandex, I have
seen him in a tiara....