You can catch the train from National Airport to Brussles Midi station, and transfer there to the train to Bruges.
The total time, including transfer, is as short as 1 hr 25 mins, depending on the specific trains you take.
Here is the website to check schedules and routes:
http://www.b-rail.be/
As an example, I picked a random date in July with a mrning departure:
SAMPLE SCHEDULE]