Assuming he was a medical professional, I'd sorta raise an eyebrow, shrug, and say 'Guess my excessive soda intake did SOMETHING right for me'. If he was some form of super mystical Marvel Universe doctor who literally said I would live for 80 more years... I'd go end wars. By headbutting people. They'd be all shooting me, and I'd be all 'Fool! My doctors gave me 80 years! 80 YEARS! BWAHAHA!', and then I'd close in and headbutt them. Thus would begin my legend.