We've got several things in common. My surgery was also in September. I stopped PT as we were heading into the Holidays' craziness, and I'm continuing at home. I also love walking, and use a fitbit. Quad tightness and soreness is one of my main problem areas I'm working on as well.
I'll list a few of the things that have been working for me. Of course, we are all individuals, but some of these things might help you too:
1) My leg is the weakest when it is behind me, like when stepping down with the good leg (when the bad leg/foot is up and behind you).
Parts of the quadriceps muscles across the hip joint as well as the knee, so it is important to stretch the hip too.
I face away from the stairs (at the bottom), and put my surgery leg behind me, tip of the toe straight down on the bottom step. I try to stand as straight as possible, and then push my hip forward a bit so I feel the stretch at the hip as well as all along the thigh and across the knee.
While I'm there, in between the stretches, I also try to push that toe down into the step, just for a couple seconds at a time (this hurts some, and I don't push too hard).
Over time, I'm trying to build strength over a wider range of motion than I have now. A regular (8") step might be too high/low, for some. I started with that, and now use a 20" chair.
2) Rollers. I use a foam roller to do the typical massage/release that runners do for the quads, glutes, side of thigh (IT band!!). Plenty of videos online. Try runnersworld.com if you don't know where to start looking. I also bought a pastry? roller (bigger diameter than a pie crust roller) at Goodwill. In bed with leg elevated, I roll out small trigger points in my thigh(s). Don't roll directly on the knee.
3) I'm reluctantly, using my fitbit to keep me from doing too much, rather than its original intent. I've learned that I can stay around 5-6000 steps with no setbacks. I still do bigger days, because I love walking and this is the way to improve my situation, but I have to take a recovery day (4K or less) after those days. I suggest learning the range that doesn't cause any problems and push against that limit just once in a while if you want to keep improving your walking.
4) I seek out uneven terrain when possible. The new knee screws with you neurologically, and building balance and stability back up is important.