If it helps at all, here's what I did in the summer of 2017. After Dark Side that year, I planned to start training for Light Side 2018 with the goal of increasing my speed for real. Every other time I attempted to increase my speed, I failed but since training would not start until September, I knew I had a few months to experiment. But as no further information from runDisney about the race came, I began to get discouraged. I basically dropped down to running twice a week, but no speed work at all. Then about mid September life got really crazy and I stopped running altogether. And once the cancellation of all west coast races was announced, I didn't have a reason to care just then.
Well, 3 weeks before the Avengers race last year, I decided to do something really insane and quite possibly stupid. Despite having not run at all in 6 weeks and not having run over 4 miles since April, I registered for the Avengers Half Marathon and modified a one month training plan to fit the 3 weeks I had. I'm not particularly fast. My PR is 3:02 and that was at a non Disney race that I set this year. My 3 long runs were approximately 6, 8, and 10 miles on Saturdays and runs anywhere from 3 miles to 5 miles Monday through Friday. I always took Sunday off. I entered the race feeling weirdly confident. I hoped that muscle memory would help carry me across the final 3 miles since I had managed 10 the week before the race. Including some photo stops, I finished.
A couple of months later, my attempts at increasing speed once again met disaster with severe knee pain in mid January and nearly derailed my hopes to finish all 3 Dark Side races in April. Well, with taking roughly 2 weeks off running altogether and returning at slower speeds once I did come back, I had time to recover and finish all 3 Dark Side races.
So that's my longer way of saying that consistent training before your injury will help you keep some, though probably not all fitness, once you return and that you have some time to get back to where you want to be for your race. You may have to adjust any time goals, but you can still finish.