General:
* Span: (especially for a geographical area)
* Reach: (especially for how far something extends)
* Range: (especially for a variety of possibilities)
* Extent: (especially for the size or scope of something)
* Separation: (emphasizes the act of being apart)
* Interval: (especially for a measured period of time)
More specific:
* Mileage: (for distance traveled by vehicle)
* Length: (for a straight line measurement)
* Breadth: (for the width of something)
* Depth: (for the distance from top to bottom)
* Height: (for the vertical distance)
Figurative:
* Gap: (for a difference between two things)
* Chasm: (for a deep, wide gap, especially in a relationship)
* Divide: (for a separation that creates conflict)
The best word to use depends on the specific context of your sentence.