Yes, there is always a race the morning of the marathon in Lexington which is a short drive from downtown Boston or also on the public transportation line. They have the race at noon I think right before a battle reenactment with cannons, guns, and everyone in powdered white wigs. The Saturday prior to Boston there is always the weekly Fresh Pond Series which includes a 2.5M race around the pond, and also a 5M race around the pond. That race is actually joggable from some hotels in Boston and is free. That weekend it may attract some good talent. Coolrunning is not updated yet with the other races, but there should be a few more. Also, try this website, New England Runner, at
http://www.nerunner.com/calendar/index.html
They don't have the April calendar posted yet but when they do, sure they will have comprehensive list. Boston area has a decent race almost every day...even weekdays. You won't find another racing town like it.