Training for long (for me) climbs

Well, I have chronic tendonitis(“golfers elbow”) and a partial torn UCL in my elbow. Can’t ride outside till 1st of June, if all goes well. So, I’ll be training for this totally inside and on the trainer till then. I will only have a month and a half to get some real long rides in and get my body used to being on trails again. Not idea for what will be the hardest day ever on a bike for me. Oh well, lean into to it I guess.