Hey, don't worry about it. Serious is good! Some of my favorite 
fanfics are serious ("Stepping Stones" by Benares and "Ranma...is 
that you?!" by S. James being two of my favorites.) Hell, I've 
written a serious fanfic in which I have Ukyou die of an unknown 
disease. 
    My advice is, don't worry about the humor. Write the story and if 
a joke seems appropriate in a certain place, use it! That's the way 
I'm writing "The Great Anime Crossover From Hell!!!" Comedy works 
much better when it isn't forced.

                                  Just a few (hopefully) helpful 
                                        words of advice,
                                        ---Erin 2.0