The key to pacing is variety. Longer, compound sentences tend to slow the pace, whereas short, choppy sentences and sentence fragments speed things up. If your character is taking a leisurely, meditative walk in the woods, you can use longer descriptive sentences to create the setting and reflective mood. But if someone suddenly starts chasing her, her mind isn't going to be on observing what's around her, but on survival. Shorten the sentences to create tension and pick up the pace.
Dialog is another way of varying the pace. Think about how people really talk. If someone is recalling a pleasant memory from long ago, they may tell it in more detail, digressing here and there, meandering through the experience. On the other hand, if someone is distraught, nervous, or scared, their thoughts and words will be choppy, jumbled, disconnected.
It is important for the writer to learn when to speed things up and when to slow them down, how long to sustain the tension and when to give the reader a break from it. If it feels like things are too slow, insert some action, either physical or mental. By alternating tension and relief, you can build to a more effective climax without losing the reader along the way. And remember that every passage, whether description or action, must advance the story. If a passage just sounds "nice" but doesn't move the story along, discard it.
One pitfall for new writers is not knowing when to end their story. Don't write past the ending. Once you've achieved resolution to the conflict, let the story draw to a quick, natural close. If there must be an epilogue, make it brief. Likewise, don't end the story too abruptly, leaving the reader unsatisfied. Even an open ending should give a sense that this part of the story is finished.