First things first: walking a lot doesn't weaken your bones, but it probably affects your joints (especially your knees)

Supplements for bones:
Calcium

(With Vitamin D if you don't get enough sunlight and Vitamin K to prevent calsification of your blood vessels due to the extra calcium)

note: I wrote a bit about calcium, vitamin D and vitamin K in this blog post. Feel free to have a read and comment if you have any questions.

But I doubt that you will need a calcium supplement, unless you have a fractured bone before.

Supplements for joints:

Most recommended: Glucosamine and chondroitin
They work well together and are quite proven to slow down the deterioration of joints.

Consider taking it together with

MSM if you have joint pain already
Fish oil - recommended for everyone anyway but in your case, fish oil is a natural anti-inflammatory which helps to improve joint function and prevent joint pain.

Other supplements:

Collagen type II
Olive extract
Boswellia Serrata (Out of all the supplements on this list, Boswellia is most proven - second to glucosamine)
Green lipped mussel

I think that's about it. Feel free to pm me if you would like a free personal supplement consultation.