I've always struggled with my knees, more before running than since though. I used to play football on a sunday, not be able to walk without discomfort all week and then play football on a sunday again. The best advice immediately is to seek professional advice because you might have strained a ligament or have something more significant going on but in my experience, knee pain is caused by overuse which requires icing, stretching and resting. See what a professional says, but I think ice and rest may be your best bet.
Thanks Ninja! It's really odd as I don't have any discomfort now, and walking, going up and down stairs etc is pain free. When I started to jog earlier it feels fine, but then the discomfort starts to kick in after a few minutes and obviously in my head this will only get significantly worse attempting 26 miles and I am fearing the worse with 10 days to go. I will take your advice and see someone, if anyone can recommend someone please do!