A WMRA-area runner was in the 2013 Boston Marathon, and much to friends' relief, he promptly said on Facebook: "I'm fine." He's familiar with the finish line area and his modestly self-described "little sliver of insight" about the attack has gone national.
His observations are in the Slate.com article "The Bomb Site Was a Stage."
In the Facebook note, he included a pre-bombing photo showing a race staging area on a street lined with school busses and barricaded with police sawhorses and a lifeguard chair (tagged, as if foreshadowing, with a NO LIFEGUARD ON DUTY sign). The bus-lined street has since appeared in a few official news photos, but after the attack and with the lifeguard chair gone.