As an avid trekker myself while that is ok, you really need to get some exercise that will stress your knees, ankles and muscles more. I am not sure which state you are in but walks like
Kokoda Track Memorial Walk (1000 Steps) If you cannot get to more natural settings up and down stairwells in highrise buildings is ok. But perfect stairs will not see your foot sliding like in will in a booth on a mountain trail.
On a treadmill you will have a regular and rolling motion. And you need down as well as up. Constant downhill can be very hard on the knees if you are not conditioned to it. my last trip to Nepal we dropped 1700m altitude in a day, and that was with some repeated up and downs and so the knees took a beating.
If you have hit your 50's like me I highly recommend trekking poles (watch some YouTube on how to use them first).
Walking for me is a glorious past-time and my main tip is to go at your own pace as it is not a race.
The Path of The Gods, Amalfi Coast, Italy
Annapurna Base Camp...and those Trekking Poles make a BIG difference. Knees are no where near as sore at days end