Barrett didn't exactly want the position at the head of the militia. He was 65 when the war started. He had his pipe and slippers ready and probably just wanted to watch game shows. Unfortunately for him, experienced commanders were in short supply, and Barrett had fought in the French and Indian war - so, colonel it was.
When the British came to Lexington and Concord looking for rebel weapon caches, troops headed straight to Barrett's farm where they were told they'd find the majority of the supplies. Of course, by the time they'd arrived most of the weapons had been removed and hidden elsewhere. Barrett wasn't there, either. He was some distance away, organizing the militia march on the North Bridge - and overseeing the eventual victory of Concord.