Tony Horton, best known for the P90X workout, says he's thriving at 66 after tweaking his routine. Horton was diagnosed with a rare illness in 2017 and has since followed a flexible plant-based diet.

Tony Horton knew he was onto something big when he first developed a new, intense exercise program that promised to trade the get-thin-quick schemes for some serious butt kicking. But he never thought ...