At very low speed, a crank drive system is the way to go, electric motors don't like low RPM, with gearing you allow the motor to rev at 2000 rpm and crawl along slowly.
Still.. for real deal 80 kms per day, it wont' be cheap on the battery side.
options include
Carry a high-power charger, opertunity charge when you can during lunch and breaks...
If the route offers the option of a pit stop half way.. swap packs (2 packs.. double the cost.. but carry only one at a time to cut down on weight )
Pedal alot and use the motor sparingly..