Unfortunately, I moved back into activity too fast.
A quick recap, with charts...
Week of March 12 (47 RHR): at my usual 47-48 resting heart rate. A normal daily up and down. The highest day of the week was on the 12th after racing a half marathon, but my resting rate settled down the remainder of the week.
March 19 (49 RHR): Average is on the move up steadily. I raced another half marathon on the 20th - you can see the rest of the week was definitely higher than normal. I idiotically failed to take a day off on the 21st or 22nd.
March 26 (51 RHR): Then ran 10 miles on the 26th. Felt punky during and after that run. Saw my avg resting rate creep up too high the next day, then took off from the 27th-30th (a period where I developed some allergy or cold-like congestion but nothing severe). Ran on the 1st when it seemed like my resting rate was back to normal, but I was only acting on one good day of data.
April 2 (53 RHR): Repeated my mistake on the 2nd, and by the 3rd, had a full blown bronchitis with fever. On antibiotics on the 5th, finally returning to a normal resting rate today on the 8th after four days with no running and no steps (beyond walking around the house and to the car, etc.)
The lesson: there's a heck of a lot of good data from resting heart rate. It's definitely a great early warning system. And it can give you an all clear after being sick or extremely run down. But be sure to give yourself a few days of evidence that things are back to normal before jumping back on the horse and getting back to activity. Right now, I'm going to make sure I see at least 5 days back to below 48 resting rate before I start significant activity again.