left knee sprain

I slept well, for the most part. While walking, as I'd run the day before, yesterday going up or down some stairs suddenly a sharp terrible pain this time in my left knee so bad I cried out. This happened while I was meeting a friend at a coffee shop. I was able to more or less walk but woke up with my knee stiff and barely able to stand. Now, a few hours later, I can walk but my left leg is still stiff. I can't .....stand to be idle. I was used to running an hour a day. I simply won't. I can go to the ER but the last time I sat there for five hours only to be told I'd sprained it and given a tylenol. I tell myself (I bought a bottle of Advils) I can read all the stuff I have with me, Vanity Fair, the book Twilight (which I'd found in Carroll Gardens)yes I'll be OK as I am a chronic worrier. But the chairs are uncomfortable so if my friend does not get back to me or if he does really I'm looking forward to re-joining the living because what else can I do?