By JAN HUBBARD GRAND PRAIRIE -- To demonstrate that he hadn't lost a step from his days as a member of the Boys & Girls Club, Jason Witten took his spot on the gym floor alongside the kids.A large video screen -- well, large for this session but nothing like the one a few miles away in Arlington --had instructors leading the group in shadow boxing, dance steps and exercise drills designed to give the kids a good indoor workout."They were a lot better than me," Witten said of the kids.