Hello. Good to have a goal like that.
Is it a general fitness concern, or is there something specific that made him say no, perhaps related to your injury?
Until I know more, I can't give any targeted advice.
However, walking (and running if you want) is the best general training for walking. Aim to gradually increase your walking time to 7hrs a day. Start with simple things like not sitting down if you can avoid it. Even standing still instead of sitting will help prepare you for a day's hard walking.
You can add things like wall sits, standing on tip-toes, heel raises or step ups while watching TV.
The steps area sounds good. If it is short then just keep going up and down fighting the boredom. Assuming 60 steps up the hill, start with running (or the fastest you can walk) 5 and walking (gentle speed) 15. Do the same coming down, limiting the fast paces to whatever is safe. Repeat for as long as you can stand it.
Once that becomes 'easy' to sustain for half an hour, shift to 10/10. Then 15/5 if you really want to push.
Then take it back to 5/15 and add a rucksack with about 5kg. Work your way through the intervals, then go to 10kg, etc.
If you can do 15/5 splits with 20kg for half an hour, you're fitter than most people who ascend those hills.