This is a fitness exercise that works with adults only, DO NOT use this in any youth development environment whatsoever.
A line of markers at the edge of the box, setup something similar at the half way line. Distance should work our approximately 35 yards, line of your players starting one of the markers and they have 7 seconds to make it to the other, when reached they have 7 seconds to rest then ready themselves to start again, and the running begins to bring the group back to the original starting point. That would be considered done twice, this routine is to be completed 8 times. Once a set of 8 is completed, the players get 120 seconds rest and subsequently start again.
6 sets of 8 is the usual philosophy but this can be adjusted based on your teams fitness levels and your own judgement.