Try bringing the knees up high when marching for a warm up. This activated the hip flexor more and helps traction the hip more. While the knee is coming up the belt is pushing down in the crease of the hip. This helps open the hip more.
Another variation is to push the leg in against the belt after raising the knee. This variation warms up you adductors more than just the knee raise. These are great if your back and hips are tight before a lower body day. Or they could be used as a finisher to a lower body day.
For a warm-up, 3-4 sets of 60 steps is good. As finisher try making them specific to the time requirements of the sport or just do a couple sets until they are miserable.