any way you can work her some over the winter and early spring ? Age wise she sounds about right, but basically having no work and then jumpin into it hard may be pushing it a bit, Endurance 101 is a new book that has come out that is really good and gets into training programs for new horses. Gearing more towards building a lifetime endurance horse as apposed to a flash in the pan. I think getting some work done over the winter even once a week, then adding twice a week, then building to 3-4 as weather improves would be fine. I wouldnt start with 3-4 days of riding. on a 3 day schedule, I would ride one day, day two would be mental, IE obstacles, dressage lesson, ground work, short putz around rides near traffic , teaching to eat/drink on command. Then the 3rd day ride again. The first year lays a foundation and is when injuries can happen. A horse gets in cardio fitness very quickly, yet it takes a long time for the skeletal and muscular system to catch up.