“I think you can handle this,” says Karima Adi, holding out a 20-pound dumbbell. The weight is thankfully not for me — I’m still using a six-pound kettlebell — but for my workout buddy Bill Cox, a 70-year-old retiree living in London. We’re at CrossFit Shapesmiths in Clapham, where I’ve been invited to join the CrossFit…