There are a couple gentle back of the knee stretching exercises you could try. Walking isn't going to get the job done for you because, well, you don't have a normal stride at this point. I did the first two and they were helpful.
1. In bed (sitting up or lying down) or seated using an ottoman with leg(s) outstretched and knee on the ottoman. Using your own muscles press the back of the knee down onto the mattress/ottoman. Hold for a few seconds or to a count of ten for a nice stretch.
2. Seated with open space between chair/sofa and ottoman, with leg outstretched and heel or ankle on ottoman. Gravity will pull the suspended knee down and stretch the back of the knee. This exercise might be painful at first and you will not be able to stand it for long. When it starts hurting too much, stop. You can do the exercise again throughout the day. The idea is to stretch, not be in agony.
3. If your bed is so situated, or you are good getting on and off the floor, you can lie on the bed or floor near a wall and put your leg up on the wall with knee straight. Again, you'll get a bit of gravity stretch (just not as much so it might hurt less) and you can tense your knee to cause it to straighten just as in exercise #1. A few members have said they like this one because they read in this position. I never tried it but it sounds interesting.