Complex sentence: Sir David served in the British Army from 1940 to 1980, including as Vice Chief of the General Staff during 1972-74 in the rank of Lieutenant-General, and is a prolific author of both fiction and non-fiction.
Split into simpler sentences: Sir David served in the British Army, including as Vice Chief of the General Staff during 1972-74 in the rank of Lieutenant-General, until 1980. He is a prolific author of both fiction and non-fiction.