1) Walk everywhere, to the store, to the park, to do errands. For example, if you are going to the post office, park far from it and if you have the kids the extra resistance in pushing the stroller or carrying them in a bjorn adds to the benefit.
2) Pick up those exercise programs likes like P90x3 and Shaun T's 10 min workouts. Focus on getting programs that offer HIIT programs (high intensity interval training) as they take very little time as little as 10 min a day up to 30 min. Both very doable. Do this several times a week and watch your fitness level come up.
3) Join a local dad's group that does regular outings such as hiking and biking. Kills two birds in with one stone. Kids get to socialize with their kids and get exercise.