Keep practicing all types of discs. Drive with putters mids and drivers. Just figure out about how far you can throw each. For example my break down is putters to about 250-275, mids 250-325, drivers from 325/350 - 400. I can throw them farther if im in an open field but those are the numbers I go by when I am playing in a tournament on a course I probably have never played.
So for you(if you throw 300 with drivers) throw putters on holes 200 or shorter, mids from 200-250 and drivers from 250-300, something like that. When you do field prac takes turns with the type of discs. I will throw all drivers for a bit, then switch to mids, and then even putters some times. As your power and form get better so will the range you want to throw each type of disc.
*edit.. Also if your on a 400ft hole and want you practice driving with mids there is no problem with just using a mid. It won't go as far, but if you aren't playing a competitive round it really doesn't matter. It will also give you a different type of second shot for that hole that your not use to.



