I do agree here, there is a difference between being started in those first 30-60 days and going into full training work.
Our home bred pack and back country horses are on mock pack strings with their pack saddles (enmpty) as long yearlings (20 months is when we normally start this) they go out in threes with one of the old baby sitter geldings. They dont go over 15 miles, and we keep them on pretty flat ground. Only breaking into some light hills when they are around two. Theh are also only doing this twice a week in the cooler months, they dont get sweaty.
We then saddle start them the spring of their 2 year old year. They will get saddle educated in the arena, then taken back out trail riding learning to use their bodies with our help on more challenging rides.
Then that winter they will be tossed back in the pack line, learn to work in longer strings and harder trails.
by the time they are 3 they generally know their jobs and what they will be doing. This is when the real work of long distance over night packs start. The rides where they are now packing some weight.
They stay sound and same their entire lives, my grandfather has been using this method as long as he can remember and it seems to be working great for what we do. Does this work for every horse? No. But for our homebreds, that have breeding we decided worked best for our needs, it does.