At the end of the ride my legs were killing me, trying to sustain 20mph on the flat/1% gradient is hard work on the bike. I've just ordered some aero bars for the hybrid eBike to see if I can make it less painful to push through the air at faster speeds.
There is no reason why you cannot have a good work out on any eBike, I'm loving mine at the moment. It gets me to work unsweaty regardless of the wind/rain, but coming home I can really smash it. If I wasn't on my eBike I dare say I would be driving to work these days rather than cycling.
Finally as they say, its doesn't get any easier, you just go faster

