what kind of karate do you teach? how long? and how long before you plan on training MMA?
in all honesty, i think the most important thing to develop prior to MMA training is endurance and flexibilty, and not go in injured. i think you're still gonna gas at first, until you get used to the different style of fighting, but having a solid cardio base will do the most for you. also, BJJ in particular is hard on people with limited mobility, so i'd suggest checking out Youtube for some stretches to do. i stretch 15-30 minutes daily (have for about 15 years), and i find it gives me a lot fo advantages when rolling.
i would suggest a schedule like this:
day 1: long run
day 2: bodyweight circuit
day 3: sprints
day 4: barbell complexs
day 5: off
repeat... and stretch a lot afterwards.
depending on your style of karate, the transition mgiht be easy for you, or it might be harder, since you'll be used to moving and striking in a different way. i would almost suggest not training until you get in to MMA classes, and just work on the conditioning.
my .02, and good lcuk!