In May last year I started exercising a pasture puff. He's also very laid back and lazy. At first our rides were about half an hour long and consisted of walk and trot. We started doing canter the next week, but only in short spurts. I rode him twice a week.
Now I ride him four times a week for 45-60 minutes. It's just a matter of taking things slow and being able to tell when he reaches his limit. I think it's also good to mix in a little lunging and hillwork.