Two components: kinetic energy (walking speed) and potential energy (fall). Kinetic = 0.5mV^2 = 0.5 x 85 x (5000/3600)^2 = 82J. Assuming centre of gravity at 1m, PE = mgh = 85*9.81*1 = 834J. Total = 916J. that's not very much, is it, enough to raise 1kg of water 0.2 degrees?

But that's only a part of the problem. It's not difficult to absorb that much energy in your shin if you do it slowly. The real problem was that the ground broke your fall, and all that energy had to be absorbed in the time it took for the flesh of your shin to decelerate your moving body to a halt, or at least that portion of your body that halted before you sprawled out and your hands took the rest of the crash.

The extent of the bruising is probably caused by internal bleeding.