I've leased a number of horses, and always seem to get the ones that haven't been in regular work for a while, so I've been in similar situations.
I find that frequent short workouts are best for out-of-shape horses. I like longeing or light riding at walk/trot for 20-30 minutes every day (if you can) to get horses back in shape and used to "having a job" again. Switching it up is a good idea, as different exercises will work the horse differently and keep him (and you!) from getting bored.
I also like the book "Equine Fitness" by Jec Ballou, which has a ton of exercise ideas for horses at a variety of fitness levels.
The nice thing about starting a horse back into work is that you see dramatic improvements. There's nothing quite like getting a horse into shape when they couldn't even keep up a canter circle when you started!