Monday’s inaugural MR8K— a five mile run and walk through Boston’s downtown and Back Bay — was a huge success. More than 2,000 people ran or walked in the event, the first MR8- branded running event by Dorchester’s own Martin Richard Foundation. The first run/walk of its kind, held to support a McLean Hospital program that treats first responders who are in need of help for depression, suicidal thoughts, sobriety and PTSD, the race ended at center ice on the Bruins NHL rink inside TD Garden.