You are never too old as long as you are open minded and willing to adapt as your physical situation changes. I suspect that for you, at the young age of mid-50's, starting backpacking will actually make you stronger and get you back in shape. Old backpackers have to be a bit more conservative about avoiding injury - either accidents or over stressing joints. Listen to your body. Pain is NOT gain. And since you are starting out today, you can get all the new light gear which is a real boon to us old folks! I am a big proponent of just get out there walking. Working out at the gym for me simply causes more injurys that it is worth. Get a dog that needs to be walked 5 miles a day!