Work to eat
Food is the best motivation to get Mittens to move, and it's also the most natural.
For starters, you should avoid giving them free access to their food all day long, as they are clearly eating more than they need.
Consult your vet to find out the proper ration for your furry friend, then set schedules and play out some clever tricks:
1. Hide the food. When your cat isn't looking, pour the food in their dish and move the dish to a different place. They have a great sense of smell and will find it quickly, no doubt, but they'll have to walk a bit first.