A very important indicator of how well your knee is functioning is how far you can walk. Many things reduce people’s ability to walk long distances such as heart disease, lack of fitness, claudication (lack of blood supply to the leg muscles), poor balance, sore feet, sore back and so on.
When your ability to walk is limited by your knees it is typically because they are causing pain for you. Instability of the joint does not usually limit the distance you can walk.
Osteoarthritis is the most common cause of pain in the knee which limits walking but injuries such as meniscal or chondral injuries can as well. The distance you can walk may vary from day to day as the inflammation in your knee varies.
Initial treatment involves simple pain killers like paracetamol along with ice, weight loss and non load bearing exercise (such as cycling or swimming). As the pain worsens you might add anti-inflammatory tablets, stronger pain killers like codeine or endone and eventually cortisone injections. Once all of that fails you would then consider a total knee replacement.