I am going to be 68 next week. Last year I walked 1150 from Sevilla-Muxia-Finisterre. It was my first camino since 2019. I have learned to actually train less then I did in my late 50's and early 60's. A few pilgrims have given you great advice.
When I factor in the amount of time I need to finish (Of course we both have the luxury of time at this point in our lives). I try to do no more than 20K for the first week or 10 days at a minimum. I also do not look at my daily camino as a 20K day. I look at it as four 5k walks. No matter how good I feel I will stop after 5k or so no matter where I am. Take my pack off, relax, eat and drink something. I usually break for at least 15 minutes. Lunch usually will be sometime after 10k depending on the time. I usually make a sandwich or have some sardines and fruit and if I am not in a town I will eat on the Camino and usually at the next town stop again and have a coffee. The most important thing you have to do is listen to your body and not a predetermined place where you have decided to stop. Going up hills I will often stop a few times even if I know I can go further. I want to give my heart a break. I have poles to push off with uphill and use the poles to absorb the weight off my knees and hips and zig zag going downhill. I slowly get my body into shape get my rhythm and my pace to where everything is a natural flow. I like to take a rest day early on my camino after 5 or 6 days maximum. If I do not do that I will do 2 days in a row of only 12 or so kilometers. As I gain endurance and strength I will walk longer distances but I always if possible keep things at 25k or less. I will still take a rest day at least weekly and do the 2 consecutive days of 15K or so. In this way I never stress my body. I never get blisters and have very little soreness in my back, hips or knees.
Also I would read carefully the tips
@davebugg gave you.
I don't follow exactly what Dave says but after 6 caminos I know what works great for me.
And remember, who gives a rat's A@$ what anyone thinks of the way you walk or how "short" your days are or how often you stop. They are not living in your body only you are. Walk your own camino. You have things you need to think about, or release or recover from. It is all yours. Start in Pamplona and buen camino.