I'm actually not sure of the math, but past a certain weight, fitness starts to decline more rapidly on its own. So you might be able to maintain fitness by doing (for example) 2 feedings and 1 exercise per day. But after a certain weight, you might not be able to keep up fitness without losing weight.
At the best, I can fatten Liora up to stage five and reliably maintain her at a good fitness level, but I have to carefully alternate between feeding and training her. At stage six, her fitness may remain good for a couple days, then it quickly deteriorates and she becomes short of breath. As an idea, when the player and Liora retire to the inn for the night, you could gain the option to purchase a "hearty meal" for her. If she exercised once or twice, but not three times, Liora will accept your offer as a reward for hard work. She will then gain weight without damaging her fitness level. If she was completely lazy or already spent the whole day eating, she will decline and explain that she needs to take care of herself a bit better.