It's very easy to calculate. On the flat with no wind, it takes about 250w to go 15 mph and 350w to go 20 mph.
The journey would take 3 hours. The battery has say 500wh, so it would be able to provide an average of 166 watts of input power, which means about 120 watts of output power (65% total system efficiency). To do the complete journey in three hours, the rider would have to provide about 130w, which is about equivalent to a normally fit rider pedalling steadily without exerting himself. That would be OK for a commute, but it doesn't allow for wind or hills
At 20 mph, the journey would take 2.25 hours, so the battery could give up its charge at no more than 222w, which would be 145w of output power. The rider would then have to provide 205w to maintain the 20 mph. That's equivalent to a reasonably fit person pedalling continuously with a bit of effort. It would make you sweat a bit. Again, that's in perfect conditions. A hilly ride could double that, likewise if you had a breeze against you. Once you go over 200w for an hour, you're starting to talk about a work-out, not a commute.
If you only had 400wh in your battery, you'd have to increase your effort by about 30w above what's calculated above.
IMHO, it's not viable to go at 20 mph, and fairly hard work at 15 mph except in ideal conditions.