Yeah, injuries are a major bummer. A lot of times with knee pain like that that comes out of nowhere, though, it turns out to be nothing serious and almost as mysteriously goes away. I was having some pain on the outside of my left knee a few weeks ago, and after a few occasions of rolling out my IT band on my foam roller (a wondrous invention for massaging away minor aches and pains, if you don't have one), it went away. So although I think it was an overuse injury caused by something being off about my cleat position and/or saddle position on the bike, the cause of the knee pain was that something was tight farther up my leg.
Of course, it could be anything from something really minor that will go away soon like mine to something really major. There seem to be a ton of different things that can cause knee pain, so definitely have a doctor check it out pretty soon. Runner's World's Web site has an article on knee pain that lists some of the causes and where in your knee it would be hurting for a particular injury, so that would be worth checking out.
Thursday, August 26th:
What the heck?! Went for a run this AM, felt fine, except my left knee was in PAIN.
This is a major, major bummer. I haven't even done anything to my knee! Why would it just start hurting like that out of nowhere?? Arggggh! This has got me super down. Aside from that I'm bloated and feeling fat and gross.
I will take the weekend off and see what happens with my knee next week. If it is still bothering me I have no choice but to go see a doctor and to perhaps delay running a marathon I guess. That really sucks but I don't want to make this worse.