The best bet for pedaling at higher speeds would be a bike specially built for that. What you need is the ability to put a 52 tooth front crank on the bike. Look for that frame clearance first and foremost. Ideally, you get your big ring without having to sacrifice a larger tire. Skinny race tires and a punishing commute don't mix well. You need a bike you can put a 2" tire on. Forget weight, if the bike that works is steel, fine.
To my surprise, I was able to fit a 52 tooth chain ring on a Schwinn beach cruiser. Stock 52t from a vintage Schwinn varsity bolts right on to a 2015 Schwinn. then I added disc brakes and derailleur. But for most, the answer may be a commuter, or hybrid bike with 700c rims. For a commute, fenders are nice.
Then, by using a front hub, or a mid drive, you can retain a rear wheel with a good cassette with 10 or 11 teeth in the back. Instead of a crappy 11t screw on freewheel, or the standard 14 t screw on freewheel on most rear hubs.
That combo should get you into the pedaling with effort at 30 mph ballpark. This is one of the reasons I did 90% of the 15 miles one way commute for 5 years with front hub. By the time I went to rear hubs on my commuter, I was riding a much larger, much heavier longtail cargo bike, and was typically running more like 20 mph all the time. I decided to enjoy the ride, rather than spend the whole time getting a right turn in my face because people can't figure out I was going 30 mph. Additionally, riding 20 mph meant I could carry less battery weight. 30 mph is great when the battery is new and perky, 3 years later, slowing down to just make it home was needed.
You can definitely get plenty of exercise with an electric bike. Your commute length is ideal for it. But it's a different kind of workout than normal bike riding. No more need to sprint, or sweat. But believe me when I say that even very mild exercise for two hours a day will make a huge difference in your heart lungs, and still tones the legs nicely. I found the e bike ride on weekdays greatly improved my ability to ride without a motor on the weekend. It also helped me ride other things better too, know what I mean, nudge nudge wink wink.
You can also fall into the trap of faux pedaling. It's very easy to faux pedal at such a high cadence you think you are pedaling. Still exercise, but it don't extend your range. The trick is to learn to feel a certain pressure on your feet. If you don't feel it, back down on the throttle just a hair. Then you can put out an easy 100w to 150w into the ride, extending your range slightly. It takes some miles to really get the hang of this, and make it so automatic you don't even think about it. But eventually it becomes as natural as regular riding is.
Here is a picture of my Schwinn with the 52 front ring. I still only have 14t in the back, but with 2000w fed the big rear motor, it can cruise all day at 33 mph, I can pedal it to about 25 mph.
is your commute flat? What kind of bike/riding?
