Is that including air drag? Without calculating it you have a massive over- estimate of power gained, as your model assumes the regen braking will be so strong that you are going 5mph downhill (where air resistance is a small factor), but in mountain biking you would be going alot faster, probably averaging at least 15mph (where air resistance is a huge factor)
I did the maths and you're right, like assuming a vertical hill 1km high and a 100kg bike + rider combo, reasonable efficiency, you spend about 400Wh going up but gain 130Wh going down with regen braking on. Not bad at all. The trick is you need a hub gear motor and extra power circuitry for regen braking so the resulting bike will be heavier, not enough to make it not worthwhile but a heavier bike with more unsprung mass will be less pleasant to ride.
Regenerative braking or some kind of dynamo would probably add a decent amount of energy for a mountain bike being used up and down hills. You'd also lose more energy going up steep hills so the 108 mile range would be reduced.
To be fair, for off-road riding that's still a respectable distance. Or, to put it another way, you'd be a sweaty mess after that distance offroad with a regular bike :-)
If you are lightweight, fit, on the flat, no or wind behind you, tyres at 70 psi, on the lowest assist setting. Otherwise expect about 40 to 70 miles