Training?
Answer: not much is required.*
Just start with a flexible schedule for your first few days.
It’s called On the Job Training.
If you can launch off with that heavy load aboard and survive Day 1, that’s all that really counts.
Do hours of lugging a heavy pack through the hills for months before you set off if you like.
Or, don’t bother.
In the scheme of things, it doesn’t matter.
*For those with a reasonable level of fitness and well-fitting boots.