About 30% of what you used to go about 30km/h will go back into the battery, but accelerating will cost more. So not using the brakes is much more efficient than regen braking itself.
60-70 km is do-able with 2 48v12ah packs. If one already gets 39 km, which I think is pritty good. Even considered you also used your legs.
the 50 or 60 km stated on the gm website is the equivelent of no stops, and only 25km/h using less then 250watt continious and optimized conditions, also no bms shutdown to prevent damage to cells.
2 battery packs will get you 60-70 km if one already gets you to 39km.
Using regen while having a fully charged pack, could detroy the controller. but I think regen is to strong. It will most likely be stronger than the gravitationel force so you will slow down to a stop. So charging on steep slopes might burn your controller, and it won't be effective.