This question can be answered in two ways.
First, it is shown that Mitty is older, probably retired; he thinks about the "weekly trips to town," showing that he doesn't need to commute for work. He is comfortable, not destitute, and married; he seems to be on the downslope of a normal, everyday life. This is not a negative or a positive; he is simply a retired man without a real goal or purpose. Since he is not lacking anything material in his life, it can be assumed that he lived a "well-lived" life in the sense that he did everything safely and correctly, and now can have his old age without fear.
Second, despite the comfort and safety of his life, Mitty yearns for something else, something exciting or new that doesn't stick to his established routine. To this end, since he feels unable to break free, he creates elaborate fantasies where he is the central figure in the world, and of utmost importance:
"We can't make it, sir. It's spoiling for a hurricane, if you ask me." "I'm not asking you, Lieutenant Berg," said the Commander. "Throw on the power lights!"
The crew... looked at each other and grinned. "The Old Man'll get us through," they said to one another. "The Old Man ain't afraid of hell!"
(Thurber, "The Secret Life of Walter Mitty," bnrg.cs.berkeley.edu)
Although he can't actually live out those fantasies, Mitty is better able to cope with his routine life because of them. He has some trouble switching between the two, but they allow him stability, as well as the ability to deal with his wife and the various irritations that he encounters. In this sense, the fantasy life is as essential as his real life, because it acts as an outlet for his frustrations and desires.