I met plenty of 80-somethings on the Camino, so I'd say you'll be just fine!
If you can only do one type of getting-ready, I'd suggest beginning to do stretching. Stretch your legs muscles slowly. And prepare by buying good equipment. In my experience those who had problems along the Way either carried too much weight, had ill-fitting shoes, or insisting on walking with injuries.
From experience, I suggest that you break the first stage from SJPP to Roncevalles into 2 days. The view are spectacular but the walking is difficult if you are not in shape. Either make reservations at Hunto or Orisson and do not let ANYONE talk you out of it. That few kilometers can make the difference between the first day being wonderful or being hell-day! We had made reservations for Hunto, but the hospitalero told us, "Don't stop! Just keep going! It's not that difficult! You can do it! It's EASY!"
I could have strangled him when I reached the summit. I cried every step from the top of that hill into Roncevalles. I was NOT in shape and it was EXTREMELY difficult for me. During the night when I had to get out of bed to go to the toilet, I thought I would die. Dramatic, yes.. but true. Once I began walking the next day, my muscles and bones warmed up, but it could have been a bad end. So ... except for that first day.. the rest is easy in my opinion. Just take caution on the first stretch and you'll be fine.
After the SJPP to Roncevalles stage, it's easy going. By the time you reach anything difficult, you've developed a pace and have built up stamina.
So... buy GOOD fitting shoes with a BIG roomy toe-box and CUSHIONED soles. Make them a size larger than you normally wear because your feet WILL swell. Carry a very light pack and remember, there's nothing you need that you can't find or buy in Spain.
Then just go... and have a great Camino.