First, to answer this question, ask yourself what traits you would consider typical of a veteran soldier. Personally, I'd think of an experienced soldier as tough, maybe a little bit jaded, and perhaps world-weary.
Then, think of traits you derive from the prologue's description of the knight. The most memorable phrase Chaucer uses here is:
And though so much distinguished, he was wise
And in his bearing modest as a maid.
He never yet a boorish thing had said
In all his life to any, come what might.
He was a truly perfect, gentle knight.
In the above passage, the knight is proscribed several traits- wisdom, modesty, perfection, and gentility. Would you compare a soldier to a maid, as Chaucer has done here? Wisdom, I'd say could certainly be gained in battle. Chaucer also indicates here that the knight has never said anything rude or crude. When thinking about a soldier at war, do you usually think of politeness of manner?
I think this description is atypical for a soldier. Gentility is probably not something I would associate with a soldier who has been through many battles. In fact, I'd think of a rough, tough manner instead of a gentle, chivalrous one as the night is proscribed.