To be given the job of commander in chief of the Continental Army was a compliment to say the least. George Washington was pretty well-known by the Congress men prior to the declaration of independence by the colonists.
George Washington had been a contributor to the militia prior to the war. He was a land surveyor which allowed him to learn about the land. This would later help him with fighting battles. He then led the Virginia militia during the French and Indian War.
This experience allowed him to be knowledgeable about fighting and leading an army. Therefore, Congress knew this about George and felt that he would be able to lead the army with his skills, charisma and personality.