Welcome to the Wonder World of Walking Horses.
Give the horse some time to find its feet. Work on the longe is not a bad thing for this. Just spend a LOT of time dog-walking. Be prepared for some other sequellae to show up (shoulder or hip pain; hock pain; mild sore back). These should pass with some time.
With Walkers (particularly from "nice" places with lots of ribbons) you can be pretty sure the horse has only the thinnest of basic training. This "lay off" time is a good time to reinforce basic lessons (stand quietly, move on command, change gait on command, whoa, etc.). You will likely find some deficiencies, here. Be prepared for them and fix them.
You've got a job to do but in 90 days you should have a good horse (assuming no permanant injury was suffered in the prior training/husbandry regime). It will take at least a few weeks until you really know what you do or do not have.
Good luck with project.