an easy program - for a distance runner type - is the dorian yates method.
while body building principles are not perfectly catered to distance running, it is a way to become explosive and regain all round fitness.
once per week, you take each muscle group to failure, which means you can't really use the muscles for several hours after the workout. for examples, you do legs and walking down the steps from the gym should be very difficult.
but you take a full six days off before you hit it again. basically there are 4 gym days, 1 hour in duration, and you do 5 light cardio workouts. you eat a lot and rest a lot.
this is what every runner should do for a couple of months per year...
year after year, the muscle fibers degrade from doing the same type of exercise.