Make sure you wear the proper shoes while walking your dog. After ACL replacement, dr. told me to wear heavier shoes (hiking boots, etc.) as they are better for your knees. Also, there are exercises that can be done to help build muscles in your legs as that is very important with assisting the function of your knees.
Make sure when you do go outside that you are walking on a flat (stable) surface with little to no incline / decline.
If possible, find an area where you could toss a ball (even if you have to keep your dog on a long lead) up and down a hill. You stay stationary while you work your dog up and down the hill. If using a long lead, you could even sit on a milk crate (or something) while you allow you dog to work up and down the hill.
Try using a flirt pole. Or even create one by using a rope with a toy attached to the end. Start playing a lot of mind games.
Use your imagination and build some sort of agility games in your back yard. Something to make your dog work harder and use it's mind. You could toss a ball over a small hurdle. The dog has to go up a board, across and then back down. Simple things that will be easy & cheap (& safe!) to put together.
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA