Start with an unladen 800. For the second run, wear a loaded rucksack—heavy, but still allowing you to move at a good clip. Next, keep the pack on and carry a dumbbell. Then, add another dumbbell. By this point, you should be loaded up enough that 100 meters is quite difficult—probably requiring a break to make it through. Finally, disencumber yourself and sprint another 800.

Additionally, practice planks for 20 minutes.

Move slowly; maximize loads. Attempt to increase loads for each of the 9 sets.

Choose a set of dumbbells heavy enough that you may need to break the 7 reps into multiple sets.