What you are doing is good.
I would do as said above, or occasionally mix speeds - you could use any phone timer and do intervals - one minute walk, one minute fast walk, etc.....anything you're comfortable with.
Adding weight, hills, uneven surfaces etc will increase the complexity and could make it more interesting.
Doesn't need to be rucking, you could challenge yourself and say "i'm going to try to carry this 10lb DB in each hand this time".....stuff like that.
The older we get, the more important it is to move consistently so you are off to a great start.