I'm going to be the voice of dissent here, but if you want to play Cassie and use less run combos, use Brawler, she has a bunch.
Then learn to run cancel because none of her run free combos hit after her overhead, and too many rely on jump in kick-mb air throw to start.
Run cancelling into b12 isn't too difficult, as b1 has such range that you can enter 'run cancel b12' as a single sequence and it'll likely hit. If you're using bf3 as the finisher, it'll sometimes whiff if you started the combo a little far out, but for the most part it'll grab the opponent out of the air with it's weird hit box (seriously, Cassie can grab falling opponents who are above her, pull them through her body and punch them to death. Don't know which parent taught her that!)