I'm thinking of The Long Walk (an early Stephen King story written under the pseudonym Richard Bachman). It starts out sort of hopeful with very dark undertones and as it progresses gets darker and darker. He makes friends, loses them. He just wants to reach the end and win the "prize"--which is that the govt will grant him any wish in the world. In the end (SPOILER ALERT SPOILER ALERT) he "wins"... but he's so completely ruined that all he "wishes" for is ... well, you'll have to read it to find out. It's quite good. And quite, quite bleak.
END SPOILER
If that's the sort of book you want to write, write it.
I rather like the suggestion, however, of keeping it sort of mysterious. Let it be bleak with a glimmer of hope but... no resolution.
On the other hand, if what you set out to write--and still want to write--is a fun romp through a post-apocalyptic environment with a happy ending, then write that. Nothing wrong with that. I know the Play Book says we're supposed to be all serious and realistic if we want to make "art," but I don't buy it. Shakespeare's comedies are masterworks in their own right and far from "realistic."
Or, how about parallel books? Ever read Milton's "Il Penseroso" and "L'allegro"? Or Blake's "Songs of Innocence and Experience"? Yours could be the novel form of that. Could be cool.
I am so glad you mentioned The Long Walk, as I loved that story, and I loved the ending. That bleak ending worked in context and can't see it ending any other way. Your ending can be bleak, if it's logical within the confines of your novel. There's no such thing as too dark, or too light, as long as it makes sense.