That's the diagnosis of what I thought was a pulled hamstring ligament. The good news is that my hamstring is fine. In fact, my doctor checked out my knee ligaments to make sure I didn't have a tendon issue. His opinion is that my ligaments are strong. Thus, this is not an injury that should stop me from exercising.
The bad news is that the cause of this stiffness and pain is that my right knee joint is aging. As I've been running marathons for almost 10 years, and triathlons for the last 3 years, the knee joint has taken a beating. It's pure physics. I'm a middle weight guy (175 to 185 depending on the training) and try to run in a non-pounding, foot rolling manner. But, the miles take their toll, and we each have a different genetic useful life of our various body parts. The body responds to knee joint stress by adding fluid. The added fluid caused the stiffness and soreness.
The good news is that after resting it for a week, it feels fine. I can continue exercising. I did a 12 mile ride Christmas day without any negative effects. I've been told to wear a light knee brace and take an anti-inflammatory before a long run. I've been told I don't have to give up doing the marathons, but to take them easier. I'll see how that goes in London in April before deciding whether I can still handle the distance.
For now, I finally scratched myself from doing the Goofy in Disney in two weeks. I haven't been able to train and I'm pretty sure doing a half marathon and a full marathon on consecutive days would re-aggravate the knee. Next up is a ski trip to Snowmass in late January, followed by the Miami Half Marathon at the end of January. I've ordered an exercise bike to help with these next two months of winter when its too cold or windy to get outside. My plan is to do more bike work and treadmill work to give less stress to the knee. Hopefully easing back in will allow the knee joint to keep from swelling. Time will tell.