The start up to the point where I need the smoke/mist animation is way over the 250 frame mark.
Is there a way to lengthen the smoke animation to be over 250 frames? Or is there another way.
The duration of all simulations (smoke, physics, particles, cloth, rigid body, etc) depends on the start an end frame settings for the simulation's cache.
For fire and smoke, select the smoke domain, then look for the cache settings of the simulation. By default the cache goes from 1 to 250 frames, but you can make the start and end frames make it whatever you want.