2 painful miles to the garden center and back. Finally got fed up with it and went to the urgent treatment center to get my ribs/chest looked at.
Had 3 chest x-rays and fortunately nothing seems cracked or broken. They diagnosed something called costochondritis which is an inflammation of the rib or cartilage that connects to the sternum that can cause persistent sharp or aching chest pain (true that!). The doctor expressed surprise that I could race last Saturday just a couple days after it happened.
He also said that I could expect the pain for another 3 weeks or so. Joy! The good news is I can run without doing anyfurther damage if I can "tolerate the pain".
Glad to know what's going on.