Hi,
I think alot of people can over do the feeding in terms of hard feed or some miracle feed. Most horses will do just fine on forage alone and light work is classed as riding every day, schooling and jumping so I think that covers a huge range of people - I would put together a fitness plan - make sure she has ad lib forage all day and night - and then do some schooling with her, as she needs to know, when the leg is on, it means forwards no questions asked!
Best of luck.x
P,s Of course some horses need hard feed to keep weight on alone and if the horse is in hard work, or depending on their breed but I think alot would do better without!