@Lucyev , kia ora (greetings, good health)
Now for the hard work
To get stamina (and fitness) I suggest people consider a training regime that works up to:
1) around 15 km (10 miles) before stopping for breakfast; and
2) around 300 m (1,000 feet) of elevation gain before stopping for breakfast;
with your pack loaded and clothing as you intend on camino.
It took me at least a year to get to that nirvana. But then I am of an age when I an no longer waiting to be called up to fill gaps in the Rugby World Cup, for example.
As a result of my training regime when it came to leave SJPdP in early May 2016 I dressed lightly and, as the way became steeper, took shorter and shorter steps (one heel just passing the heel of the other foot) and breathing in rhythm with each foot fall. Even so I was one of the earlier arrivals at Roncevalles.
My training is intended to make walking 20-30 km day after day common place with all gear bedded down, so, come your camino you are quite comfortable and enjoying each day and have energy, and time, to spare to look around at whatever might take your interest.
Kia kaha (take care, be strong, get going in 2020)