[ZenGum]

Regarding adjusting your calculations for rough terrain: I say, don't.
I presume that you're not really concerned with the distance you move your body. Your real concern is getting exercise, aerobic stimulation, cardio-vascular workout, burning calories.
A three foot forward / one foot upwards pace on a hillside trail most likely delivers more of all of these things than a five foot pace on a flat footpath. More so if you are carrying a load.
By and large, the things that cause you to shorten your stride also improve the exercise value of each stride, so I suggest, just count all strides as being unladen 5 foot strides.
I guess an exception would be if you are walking gingerly on an icy footpath or something.