Smilodon, I just did my last revision class today, did 8 of them recently, on about 14 different topics. I found prep time actually to be quite minimal. The key strategy was: It's them who have to learn the stuff, not me. Make them come to the revision session "having already started revision". Then what I did was: Give them 2-3 questions similar to possible exam questions. Have them choose one. Have them devise an answer plan within 15 minutes. Then have them discuss their plan with their neighbour. Then have the pairs present their combined/revised plan to the class. Have other students comment on it. And times up.
I hope you can find a similarly effortless way to conduct your revision classes!
I did variations on the theme by having them devise a "possible conclusion" or "possible introduction" instead of an answer plan.