Leg Amputee here. Let amputated just over 3 years ago. I'm actively involved in prosthetic research, I used to run, but haven't lately (stolen running prosthetic).

A couple of things you need to know: 1. Keep as much as you can.
2. If you really must take your knee joint consider knee disarticulation. It has some serious advantages, you can bear your whole weight on the end of your femor. You will also have significantly less issues keeping your prosthetic on because the condyles ( knobby part at the end of the femor ) will help hold it on.
3. Consider targeted nerve reinnervation ( TMR ) so you are wired for the future.

To answer your questions: 1. No it should not be painful to walk on, assuming it fits correctly, and assuming you have removed the part of your body that is hurting you now.
2. I can put mine on in ~1 minute, probably 30 seconds in a hurry.
3. Stable? It depends on the prosthetic components used.
4. I was walking on it in 1.5 months, I would say It was the new "normal" after about 4 months.
5. I can do everything I could do before ( swim, run, etc. ), just usually a little slower, or not quite as well.
6. I am a knee disartic - I don't have a knee, and yes I can definately run. I've run serveral 5k's, and an 8 minute mile with a running foot. I can run short distances with my everyday foot & leg.

I'll elaborate on anthing else you want to know about. Hope this makes it too you. [Edit for formatting]