As a soldier, Mark Valley fought Iraqis in Desert Storm. So taking on the rabid fans of NBC's Days of Our Lives shouldn't faze him. But Valley plays Jack Deveraux, replacing actor Matt Ashford, whom some viewers still clamor for more than a year after he was fired. Valley, a New York State native, beat out numerous ''names'' and began appearing last week, though his only previous soap experience was five episodes of Another World. ''To tell you the truth,'' he says, laughing, ''I have no idea what I'm getting into. What pressure?'' That attitude's no surprise, coming from a West Point grad and former Army parachutist. He says the military and soaps have much in common: ''You get up early, you need discipline, and you never know what to expect.''