Hm, sorry. Sometimes I nest thoughts together strangly and they come out as a mess when I type them. Let's try this again.
I meant to say...when I write, there's two types of pacing problems I can have.
One type happens within a single scene or chapter, where the action just keeps going and going and going, and because the sentences are short, it runs together like you described in your original post. In the end, you have a scene that fits on 1.5 pages, but the same scene, if it was written by another author, might take up 4, or 10 pages. The 1.5 pages, since it's so short, goes by very quickly when a reader reads it, too quickly for the reader to process.
The other type of pacing problem has to do with the overall story. IE, maybe you have the first two chapters down, and they read ok when you stop to read them, but the next three chapters drag on, before you hit the middle of the book where a lot of action picks up again. Those three dragging chapters might be considered a pacing problem, but it's more of an overall-story pacing problem, as opposed to a more "local" pacing problem. Make sense?
Addressing the first type of pacing problem, the "local" or "intra-scene" pacing problem, where short sentences run together...I brought up the "show don't tell" rule, and narration. "Show don't tell" is a common writing "rule" that gets kicked around writer's circles. A lot of new authors write about things that happened "off scene" in their story, instead of "showing" the reader what happened directly. This tends to make a story rather boring, because you are "hearing" the story "second hand", instead of "watching as it happens". Sometimes that rule gets pounded into a writer's head too firmly, and they end up "showing" everything, with little to no narration. This can cause pacing problems.
"Telling" is narration. It's the text you write when you say, "Thirty days ago the storm killed my city. It crawled out of the sea like a stringy underworld kraken and devoured everything I knew like a starving dog cleaning out its kibble bowl."
"Showing" is action. It's "I watched from the second story as the storm surge rose up and breached the wall, hitting the concrete with a slap. It scared me, and I trembled near the window, wanting to hide, but too frightened to look away."
If you have too much "showing" going on, and your sentences are short, you end up with a laundry list of events that happen. I've found that as a reader, I start reading faster and faster and faster when I've managed to do something like this, and it totally screws up the pacing. If you have something like this happening to you, you might be lacking bits of narration, where you step back from the action for a moment, and insert some sort of thought or summary related to the scene but not directly involving step-by-step action or dialogue, to bulk it up.
When I'm having this problem, I go back in, and between lines of dialogue or actions or events happening, I insert opinions that my point of view character is having. Say the character is running down the street after an enemy. You might pause to note that one of the neighbors plants marigolds in the cracks of the sidewalks, and that your character feels it makes the neighborhood weird, before going onto the next action, where the enemy pulls out a gun from his pocket. Inserting this little note about the sidewalk or something your character notices or some opinion on a subject that your character has pads a scene and bulks it up. It breaks up the action, and bulks the scene up so everything isn't just happening on top of everything else. It spreads out the events that are happening so the reader can catch their breath.
I don't know if this is what's happening to you, but I've had it happen to me, and this is one of the things I do to combat it, which is why I'm throwing it out here...
Anyway, here's hoping I make some sort of sense this time.
I'm being fairly abstract, so...::shrug::