Blood River Down — Charles L. Grant

Gideon Sunday, an ex-semi-professional American Football player, goes to his pantry to retrieve an awful bottle of his (dead) sister's fruit preserves. When he gets there, he discovers a meadow in his pantry. He closes the door, takes a few breaths, and begins to investigate with as much sanity and candor as Sherlock Holmes. Upon discovering that A) no one is playing a trick on him, B) there really IS a meadow in his cupboard, and C) there is something three-quarters of ugly coming out of it, he decides to beat a monster to death with a baseball bat.

Upon his conquest, he meets the lovely, and helpless, and possibly stupid Glorian, who tells him that she has come to get him so that he can fulfill a quest. Gideon, you see, must find a white duck and bring it to a certain river and put its down on the river to keep the land from being flooded. No one has really seen the duck, no one knows what is so special about the duck, no one knows where the duck is (supposedly). Glorian just knows that Gideon has to get it. Why Gideon? Because that is where her magic portal took her.

Along with a bellicose young man, a retired weapons trader and master, a dashing and dangerous young lady, and a large, peaceful goat (and a band of singing thieves, like Robin Hood, except with song), Gideon has to break in to the most heavily guarded fortress in the world to rescue Glorian, steal a duck, and save the world.
Yes. It's that awesome.