What
@Nova Haibike said. I'd like to add to that. Lightweight, check. No wind, check. (Tail wind, +1.) Gentle terrain, check. Low level of assist, check.
Add in: Low speed. If you go over 30 kph, the motor has to work progressively harder to overcome air resistance, draining more battery. It's a rapidly increasing curve, such that the increase from 30 to 40 kph will take double the watts than the increase from 20 to 30 kph (rough estimate. YMMV.) So to get 200 km, you might have to go 15 to 30 kph the whole time.
Lots of hours in the saddle, not counting time off the bike for filling up water bottles, grabbing a quick bite to eat, rapt contemplation of the wonders of nature, etc., makes for a long day.
I used to do those kinds of days when I was younger, fitter, pre-COPD, and enjoyed what I gather they now call bike-packing. I called it bike camping. Honestly, if I still had the legs and lungs for long days like that, I would use a lightweight touring bike instead of an ebike. Just sayin'. I totally get the appeal of ebikes, and can imagine wanting to use them for really long rides. Maybe there's a place on the continuum between that younger, fitter me and the current, older, health-challenged me, where the tipping point would have been long days on ebike or no long days at all.