Dan Walker is a freelance agent for the CIA.
He is actually working as art director for a large company in Denver as the two-book series begins, happily living a normal life without danger but also without excitement. He had been doing so for at least ten years and had no plans to change that until a call from an old friend in the Agency asks for help. Without so much as a blink, he flies from Denver to a Texas border town to meet his friend and suddenly he is in the thick of things. It is likely the boredom that he felt with his job made it not too surprising that he took the first assignment with very little argument.
Walker has served during WWII in Naval Intelligence doing clandestine work with Major Fredricks as his boss and his friend. He stayed for a short while after the war ended but eventually left the service for the private sector when budgets were being slashed and personnel were no longer needed. It had been a decade since he had last seen Fredricks. From the sound of things they had kept in touch but by post and phone, not in person.
Then a call by the Major with the request to fly to meet him in El Paso and the request to help the agency out on a mission which needed an experienced agent who was not formally connected to the government. Dan Walker fit that category.
Walker is just at 6' and in fairly good shape, good enough to be desired by several women he meets but not so great that running and climbing during his first adventure does not get him winded and regretting his lifestyle. He is certainly in his late 30s or early 40s when the series set out. He can handle himself in a fight but is smart enough to try to avoid them, which is the same attitude he has towards guns.