I wouldn't be surprised at all to see a foot race at the final pitstop. Other teams have weaknesses as well; it's not like everyone else is an athlete and that Charla is the only one who can't run fast. The "brother" would probably lose out in a footrace also, because of his bad knees.
The great thing about TAR is that it takes strengths in many areas, not just physical ability, to succeed. If a physically-challenged team really wants to win, they have to outsmart the other teams early enough in the last leg to get a decent lead on them and avoid the footrace in the end.