"Fitness for young people is an option, fitness for old people is an imperative, " Bortz said.

CNN: The new face of fitness is getting older

"Obviously, pain is nature's signal, but you can't be a pantywaist about pain, " said Bortz.

CNN: The new face of fitness is getting older

Dr. Walter Bortz, author of the books "Dare to be 100" and "Living Longer for Dummies" is 77 years old, but he has run a marathon every year for the last 35 years.

CNN: The new face of fitness is getting older