Although our knee is a large joint which has a lot of weight as well but its motions are not as much as other joints in our body. It can bend to almost 120 degrees and supports the leg. It can not move in just any direction. The strength of the knee comes from its limited motion but it is not made so that I can take a lot of stress.
The knee stretches to a straight leg and it it needs to be stable. In case it can not stretch fully, the muscles need to support the body all the time and that puts a lot of continuous strain. We can relax usually when out knee locks in the straight position. The horses can sleep standing upright because they rest on their knees. If you wobble knee side to side a lot, that causes a lot of stress on the side ligaments and it might lead to a lot of problems.
There are two compartments of cartilage in our knee. One is the inner cartilage and the other is the outer cartilage. In case the cartilage is even, the leg can bow in or out but in case you are overweight, then you are bound to have a lot of knee problems. You should not do things that will increase your pain. You should use a can as it is helpful and also an elastic bandage in case of any injury. You should avoid using a pillow near the knee as it might cause the knee to stiffen and not straighten out in the night.
You can start with exercises and can repeat them many times a day. Swimming does not involve any weight lifting, so that can be done as well. When you start, focus on straightening the leg and flexing it. If you have a help to move your leg and help in movements, then it will be better rather than doing it alone yourself. The bottom line is to work at getting your leg straightened. You can then go for isometric exercises. You need to tend the muscles in your upper leg, front and back so that your leg does not have to move but there is an acceleration of force. Do this for two seconds and then stop for two seconds. Do this ten times continuously three times a day. Then slowly begin active exercises. Working on a bicycle with a low gear is a good starting option. Stationary bikes are a good option. You can start walking as well as this is a good exercise. You can increase the distance gradually. Do not do exercises that put a lot of stress on the knee.