looking for a progressive preseason conditioning program

One of my favorite pre-season (and occasional in-season) conditioning drills is what we called "Terrific 20's". It goes something like this...

Separate team into two groups--Rabbits and Turtles ("skill" players and "linemen"). A leader or two is appointed for each group. Start at a goal-line, group on the goal-line (two lines, if needed) with leaders out in front. Start with 20 pushups--leaders calling it out, run down to the opposite end zone, and with the full group, do 19 situps/crunches with leaders counting out. Run back, to 18 pushups, 17 situps, all the way down to 0. The team will end up doing 2000 yds of running, 110 pushups, and 100 situps. We used to put a time to this--14 mins for rabbits, 16 for turtles, for example...whatever you like can be used. The kids honestly don't complain about these too much, because they are doing things as a team. It is not a drill that "mimics" the game, per say, but it will show you who will dig down and work. If you don't think your guys can handle the full 20, have them run 10 or 15. I have yet to find anything that speeds up conditioning like this. After a round of T-20's, suddenly a handful of 20's or 40's don't seem too bad. Before you know it, the kids are running their other conditioning harder and faster because they know they can make it.