Knee pain help!!??

I run about 20 mpw avg. and train for multi-sport. I have noticed that when I run more than about 8 miles one knee hurts just below (inside) the patella. It's mostly bearable but does get very aggravating sometimes. I haven't ran a race longer than a half-mary and want to start marathon training. I'm worried that it will be too big of a problem once I start pushing beyond 10-12 mile training runs. I ran 14 miles 3 days ago through a lot of pain and it's bothered me today still on a 6 miler. It doesn't bother me on the bike or swim unless it's sore from a long run the day before. It's not ITBS (I get that too sometimes but know that's overuse on bike) it's more on the front of the knee under the cap. Is this common? What can I do?

Please advise...thanks, Rich