Your problem isn't pace. A one thousand word short story can be very slow paced, and a one hundred thousand word novel can be very fast paced.
Too short usually means you're writing too narrow, the same way you would write a short story. You have to widen the entire novel right from page one. I'm not a believe in using subplots to add length. One subplot that is connected to character and story is good, but after this, most subplots are just padding that take reader away from main story and character.
Add a character, add complications, and add scope. This is how you make a novel full-length without padding. Widen the view, and make sure you aren't writing anything, from characterization, to description, to complications, the same way you write it in a short story.
Many an excellent short story has been turned into an excellent, and long, novel without adding and padding at all.