It all depends on how fast you go and the grades. The REx engine is only around 34Hp. The motor can draw at max 170Hp (but not for extended periods). It doesn't take a huge amount to maintain a steady speed, but the power required goes up at the cube of the speed. A little difference makes a much bigger difference.
FWIW, while this is only true on a serial hybrid versus a parallel one, the vehicle ALWAYS runs off of the batteries. The REx just is putting some power either directly into them, or it's going to the electric motor if there's not enough. If you let the batteries get quite low, you will then be running directly off of whatever the engine can produce...think 1960's, early VW bug...not very fast! The battery, when it has enough charge, can sustain much higher outputs, but not forever, letting the 170Hp electric motor reach its full potential...exceed that with a low battery, and it's only whatever is being produced as the battery can no longer act as a momentary storage/augmentation for that power draw.
You may need to plan to stop more often than 70-miles if you let the SOC drop quite a ways. FWIW, people have taken the i3 on much longer trips, but it can take some planning and restraint. It is designed as a city/commute car, not a long-distance cruiser.
SOme parts of the country, especially at certain hours of the day, may not have fueling opportunities every 70-miles, so if one is unexpectedly closed, your choices drop radically. On cruise, my ICE can go over 500-miles, giving you lots more opportunities, but it does use more energy in the process. I use the i3 most of the time, since I don't need that range. But, I don't take mine on longer trips.
2011 535i x-drive GT, 2014 i3 BEV