Confidence in what you can and can't do, comes from
doing the exact thing that you are worried about. I do not know when your pilgrimage will take place, but if you have time pursue walking with your backpack on, carrying same approximate weighted load.
Confidence Building and Fitness
Start with small distance goals. As you reach that goal of a set distance and are feeling good about that distance, then you can increase that distance a bit more. Shoot for one-week targets, those targets can be any incremental distance from 1/2 Km to 5 Km. As your feet, ankles, leg muscles, back, and cardiovascular system adapt to one distance and you feel reasonably good, either go a little faster over that same distance, or keep the same pace but add more distance. Do this in a goal you can reasonabley reach in one week's time..
Now, focus on getting muscles and cardio used to longer uphill stretches. NO HILLS?? No problem. Use a treadmill set to about a 6% incline. You will set the walking speed to as low as needed to slog along with your backpack for 20 minutes. Take a 5 minute break to rehydrate, then do 20 more. Slow the walking speed if you need to, but this is enough. As you get fitter, you increase the incline a bit more NOT the speed. When your treadmill hits a 14% incline and you can maintain the 2 x 20 minute walks, you THEN increase speed a bit and go from 20 to 25 minutes.
The eventual goal would be to continuously walk at a 12 to 14% incline for 1 hour without being exhausted. Continuous sip of water as you walk on the treadmill.
Distance Walking Technique -- Slow is Smooth and Smooth is Fast.
At the beginning of a day's walk -
and after nice, resting breaks -- your body will naturally feel good and your instinct is to walk faster
because you feel good.
DON'T. Keep your pace at a speed where you can maintain for a longer walking period of time. That means start out and maintaing a much slower pace than you feel like you can do.,
Basing your walking pace/speed based on how you 'feel' or by trying to keep pace with other people that are faster walkers, shorly leave you gasping for air.
Resting Breaks
Do you know that there is a name for your stated fear? And that YOUR concern is shared by all professional and amateur athletes, backpackers, climbers, marathon runners, etc.? In other words, you are NOT alone in your concern.
In America, we call this phenomenon '
crumping' -- it is a point where your body has hit a Wall of exhaustion so hard that the only way to recover is to stop and rest for the night. Your store's of muscle glycogen and sugar conversion are depleted to the point that your Kreb's Cycle has two, flat tires.
So, a hard rule to walk longer is to not overexert yourself while walking. If you go into an energy debt repeatedly, your day will end far sooner than it needs to. Always pay attention to how you feel. Plan your breaks at intervals where you are not
forced to take a break.
At my top fitness level as a backpacker, I liked to pace myself to walk for 55 minutes at a stretch. At 55 minutes, regardless of HOW I felt, I would stop for 5 minutes for a short break. After four of those stretches and breaks I would stop for a meal and rest for 30 to 40 minutes.
Then wash, rinse, and repeat up to three times. How far I made it on the trail or on a Camino route was incidental. Of course on Camino, if I feel like stopping for the day early because I want to stay the night at a particular village, explore some wonderful sights, or just because I want to I have that choice.
When backpacking on the trail in wilderness, my destination for the night is wherever I'm done for the day. My 'House' is already on my back.
Energy Maintenance
Eat the equivalent of 1/4 of a Snickers bar (easy to carry, or something similar) and a small amount of cheese, like a string cheese about every 20 minutes with sips of water.
A SMALL amount. It takes the body about 20 minutes to replenish what your muscles expend, so small niblets every 20 minutes or thereabouts, will help reduce fatigue.
Water and Hydration
I am in favor of water and hydration. People use bottles, water reservoir, hybrid solutions.... none of it matteres if you do not stay hydrated. If you are seating and your pee has a distinct yellow color, instead of a light straw color or clear, you probably aren't drinking enough. Especially when sweating.
There is a lot more information to guage the amount of intake for water, but thirst is when your hydration level guage is getting really low.
I am not a fan of additional electrolytes and such, unless there is extreme activity and the sweat is literally pouring off the body. Meals and snacks are frequently more than sufficient to provide the needed electrolytes when walking. As a combat medic in Vietnam where humidity and heat and slogging 90 pound rucksacks was part of the job description for our Infantry and Marines, extra electrolytes were required.
For Camino, extra electrolytes MIGHT be an individual thing to assess a need for. As a
routine thing for everyone as the norm, that, in my view, is a bad practice and does have potentially hazardous effects when adopted as a routine.
Miscellaneous, Just in Case
There are posters for the telephone numbers of taxi companies stapled in surprising places all along the Camino: fence posts, utility poles, other signs, etc. Taxi numbers will usually be posted in the bars and tiendas and mercados when you stop at for refreshment.
Make it a habit to snap a quick shot of these as you walk during the day just in case. Don't ask me how I know to do this