Posted: 12/04/2013 at 06:21
Are you working to a defined training schedule for your marathon training? People will often warn about adding too much weekly mileage too quickly - I certainly find I get injured if i do that so it could be worth checking you aren't over doing it. Adding no more than 10% extra milage per week is often quoted as a rule of thumb.
After training, for me the next biggest factor for my knees seems to be running shoes. If go along to a decent running shop and take your existing shoes and explain the problems you are having they might be able to recommend a suitable pair for you.
Good luck with the training