I doubt DHS was even involved in the Boston marathon security and I know for a fact that you'd be peeing your pants in fear if they had been.
This is correct. The DHS played no part in security. Such security duties are left at the state and local level. They responded immediately and began assisting the Boston Police Department in their investigations, along with the FBI, National Guard and state police. In my city at least, security for large events is provided by the local police department, and a few state police officers. You don't see anyone from the National Guard or DHS there.