In principle, you would use about 100W (your own energy) for moderate pedalling at 22km/h. If you select 300% assistance (high), that will add another 300W and you can pedal almost twice as fast, i.e. about 27km/h (you will encounter increasing resistance from wind). Therefore, if you got a 400Wh battery, you got a range of about 37km at 37km/h. If go more slowly and use less support (200%), then you can drive further, e.g. 60km with a 400Wh battery. (Read more in die Die Schnellen im Vergleich)
Photo: This typical electric bicycle, a Sanyo Eneloop (now discontinued), had a range of about 30–55 km (17–35 miles) and a top speed of around 24 km/h (15 mph). Note the 250-watt hub motor on the front wheel and the 5.7Ah lithium-ion battery pack (black, marked "Sanyo," just in front of the back wheel). Picture by kind permission and courtesy of Richard Masoner, originally published on Flickr under a Creative Commons (CC BY-SA 2.0) licence.
There are individuals who claim to have lost considerable amounts of weight by using an electric bike. A recent prospective cohort study however found that people using e-bikes have a higher BMI. By making the biking terrain less of an issue, people who wouldn't otherwise consider biking can use the electric assistance when needed and otherwise pedal as they are able. This means people of lower fitness levels or who haven't cycled in many years can start enjoying the many health benefits E-bikes have to offer.