I work with a fellow who's (a) quite an athlete himself and (b) has a masters degree in exercise physiology. I decided to hit him up for some info on my possible injury, and here's what I got back:
"Sounds like a tendon strain or, at worse, a partial rupture. Could be an acute onset of a chronic condition, in other words, standing up caused an acute insult to a chronically strained quadriceps tendon. Chronic strain could be caused by a sudden increase in repetitive functional overload concurrent with a sudden increase in training volume and/or intensity. It hurts to touch and on extension - is there any swelling? I'd agree that ice an rest (and light stretching actually) would be indicated at this time."
His identification of a "chronic condition" rings a loud bell with me. I know that for the last two weeks, my entire right leg - from hips to toes - has been amazingly tight. And the two weeks before that, it wasn't much different. My lower back is sore and stiff, I get kinks above the right hip, my hamstrings feel tight, and my calves have little flexibility (well, ALL of me has little flexibility).
So the rest of this week, I stretch. I stretch after every workout. I stretch in the evening. I stretch, stretch, stretch! If I'm going to do this marathon in May, and if I'm going to keep doing this long-distance stuff, I need to regain (newly acquire?) some flexibility. That's Key Point Number One this week.
And with that, I'm going to try to continue the training. Swimming should be okay (I may need to avoid 'fly and breaststroke due to the kick and my knee), and I'll need to go easy in the bike and run at first. Some short playing-around runs felt fine (even the ones with my daughter on my shoulders), and with the return to some squat-like mobility without pain, I think the bike should be okay too. I just need to be gentle at first, and make sure I don't make this tendon issue a chronic one.