The "runners" you refer to are probably floor joists, based on the fact that you say they are 24 feet long. Your specific question is if the floor joists are OK spaced every 48 inches, rather than 24 inches.
I am not sure if you are asking this question with respect to code (does my garage meet code?) or are you concerned that the garage may fall down because of inadequate number of joists?
The question of how much load you can put on the floor is a function of the bending capacity of the floor joists, which at 2x6 spaced every 24 inches, spanning 24 feet, is very low. To give you a more precise answer, we need to know the exact dimensions of the 2x6 (are they full size or dressed?), and the maximum allowable fiber bending stress of the floor joists, which may be marked on the wood, or could be estimated if you know the wood species and grade of wood.
Perhaps post some pictures, and try to determine the species of wood. And let us know if this is a code issue, or for your use only.