Hi
@AlexanderAZ, I'm not sure I can help much, but may I suggest some lateral thinking. I assume you're planning on a big section of the
Camino Frances.
I live in Australia. I walked my first Camino last year (2016) - 800kms of the CF. I trained for a year before heading to Spain. But I skipped the hottest weather, both during training and on the Camino. I started walking last few days in March and got to Santiago not quite a week into May. Spring in Galicia was paradise.
My toughest day was the first - walking up to Roncesvalles. Probably a combination of nerves and gradually coming down from an adrenalin rush after the first few hours. The Napoleon Route was still closed - jammed with snow. There was also snow up at Rocesvalles, but after walking a couple hours down from R. I got clear of the snow and never saw any again. Although it was zero degrees centigrade (=freezing) when I was in Burgos, so that was a near thing.
The descent that I found the most strenuous is down towards Molinaseca, just before Ponferrada. You'll be fit by then but I strongly recommend walking poles, I found it treacherous - it was wet. Alto De Perdon was not as difficult as I expected but it's the round stones in eroded gullies that make things difficult on the worst descents. Your ankles will take a hit every time all your body weight lands on one ankle at a time and the ankle pops to a new position because the rocks are round and unstable.
My first Camino decision was not to walk in summer. I thought my feet might sweat and blisters might happen. It's easier to control body temp with layers in cooler weather than to be stuck in the heat, as you know better than most.
I think that overall fitness is most important. I have a small exercise machine which is like a rowing machine. I was able to train walking using walking poles and carrying a pack - did that for the final couple of months. (In ALL weather. You'll never get used to walking all day in the rain but it's useful to experience it a few times before you get out there.) Using walking poles properly takes a lot of stress off your legs - going uphill it's like having 4 legs! Poles are essential going downhill - for stability and safety as well as protecting ankles and knees from shocks. Without training your upper body you won't be as ready as you might otherwise be. That would be OK a couple of weeks in, I expect.
Feet are a whole other issue. I never got a single blister but that's basically a genetic thing regardless of what anyone says on the forum (Google "skin shear, blisters"). I have weak arches and a weak ankle so I got a bad case of plantar fasciitis on the Camino. But everyone is different - this is never more true than regarding feet. I have gathered info about strategies which will help me to control plantar fasciitis if it rears its ugly head again. A major strategy re feet is to give yourself an out - stop half way and go back the following year, for instance. Look out for people who say "just wear such-and-such brand of shoe/boot and all your prayers will be answered." You have to discover what suits you specifically. It's a complex issue and if you accept that, you'll be open to learning more.
There are different stretches for calves and achilles/feet. For arches: stand on a step at home with just the balls of your feet on the step, then slowly lower yourself down an inch or so, hold, slowly up to start position, hold, repeat. Difficult but over time, it will strengthen your arches which is where the stress is while walking. If your legs get stiff and sore (duh) get a massage once in a while and maybe take magnesium supplements for muscle soreness. Take regular days off - you're not a hero, an Olympic athlete, or a machine. Stay hydrated, you need more that you think.
I was 72 while walking last year, so some might think that a geezer's experience might not apply. I disagree. Future strategies for me include walking max 400kms. Others can walk 1000kms and turn around and go back, maybe you can too. Lucky you. I have learned to accept that I personally can't do that.
Some people will say "Just go. Don't overthink it." I suspect that means "God/The Camino will take care of you." I take care of me.
Any questions, please contact me via a personal conversation, no problem. You'll almost certainly get a lot of conflicting advice here on your thread. Everyone wants to help but you need time to discover what suits you - physically and other ways too. I'm not a machine and I assume you aren't either.
Buen Camino, Have a wonderful time. I did and I'm planning my next trip(s) now.
Mike
! Bring the ramp and the mud simulators along. 
