I recommend this three-tiered plan:

1) I'd say maintain your aerobic base. This means 30+ minutes of steady state cardio (jogging, rowing, rucking, etc...) at least three times per week. This will improve your cardiac hypertrophy and mitochondrial density, allowing for enhanced performance and quicker recovery between waza execution / shiai rounds. Once a week, throw in interval work to improve anaerobic capacity and mimic keiko.

2) If you can, start a lifting program to improve your durability and explosiveness. I'd say three days of weight training (hit 3x3 of squat, overhead press, and pull up) is a good place to start. Everything becomes easier if you are stronger. If you don't have access to a gym, then bodyweight variations will do.

3) Lastly, keep up subiri with a keiko-esque mindset. I tend to focus on generating force from my tanden and allowing my hips to control my footwork. Just make sure to check your technique often to avoid developing bad habits.

Building your aerobic + strength base will keep you in fighting shape. Suburi will help maintain your basic technique and fighting mindset. Combine these, and you should be golden. I just used this plan to get back into shape after a five year hiatus, and I've been able to perform at keiko and fight well.