Is it normal to have knee pain after long runs?

It's pretty normal to have sore knees after long runs, especially if you're not used to that kind of strain. It could be muscle fatigue or inflammation, usually nothing serious. If the pain sticks around for more than a few days or it's really intense then yeah you should probably get a doctor to check it out, but most of the time it goes away with some rest and maybe some ice :)

It's normal but it really depends on how you run and how trained you are. I personally had an interesting experience with this, because when I started jogging three years ago (I was sedentary at work, spent 8 hours a day walking around the parking lot of the complex where I work), the first few months were terrible. My knees hurt, my back, everything. But I discovered that the problem wasn't running itself, but that I was running badly, like with old shoes and without doing proper warm-ups. When I invested in real running shoes and started doing stretching before and after, everything changed.

What I'm trying to say is that some fatigue is normal, like I said above, but if the pain is sharp or lasts for days, then maybe you need to change your approach. What helped me the most was increasing the distance gradually, not trying to do 10 km overnight. And then this weird advice a personal trainer friend gave me: do some leg strengthening exercises when you're not running. I often, while I'm sitting in the evening doing my craft projects (I'm making a macramé organizer right now), do some isometric exercises for my knees. Sounds silly but it really works.

If you feel pain after a long run, try ice for the first few days and maybe rest a bit. If it doesn't go away, then yeah, consider seeing someone, but in most cases it's just a matter of adaptation and running technique.