What types of things can i do to get these horses back into shape?

Well if they haven't been ridden in awhile I would start with ground work. Lunging on a line or in a round pen is always good, or turning them out in a dirt arena works too. Start with walking and turning, then trotting, just work your way up through the gates. Also, reducing some of their food or maybe lower calorie/ fat food will help a lot. If you are nervous about getting on them but you want to introduce them to trails, just walk them on the trail in hand, slowly building the distance from the house. Emphasize the groundwork before you start riding and the horses will respect you and will be willing to work. If you get the horses out almost everyday, they will quickly get back in shape!