How fast was the 42k, compared with your normal pace? I ask because I have to limit both the distance of my long runs (currently training for my first marathon) and any interval / speed sessions I do, due to the effect on my knees. The main thing is probably to give it time to settle -might be a week or two- or, if you can afford it, get it checked by a decent physio / sports therapist. I have found from experience that knees are quite complicated things(!!!) and pains can result for any number of reasons. (previous root causes for me include: inappropriate running shoes, dodgy hip/back from rugby, ITBS, hamstring problems AND runners knee. All can be resolved, but the cause and treatment is different in each case.
If it doesn't settle soon, get it checked anyway.
Hope it is soon ok.