I am probably fit enough to still ride a non assisted mountain bike, and still do, but quite occasionally at the moment.
I initially got a hub drive road bike (Oxygen emate) to ride the 20 mile return trip to work, and for me it was the ability of an electric bike to carry two loaded panniers with stuff I needed at work, without slowing me down too much that made commuting to work and back on a bike possible.
The two towns I commute between have superb off road options and after getting from my house to the first track I can be off road almost all the way. That persuaded me to get my crank drive Yamaha Haibike in 2015, add a rack to it for the panniers and enjoy some great countryside and tracks. On thursday my off road route was 12 miles and used 31% of my 400Wh original battery. Theoretical range using eco and off 38.7 miles.
I don't use the Haibike for anything but commuting to work and back, 12/14 miles off road to work and 10 miles back on the road. Over 11,500 miles so far.
Now it does not matter if I get sweaty riding to work as I can sluice myself down at work. So my ride to work is an exercise ride, using eco and off where the gradient allows and getting a work out enjoying the countryside.
Going home is a road ride later at night and I want to complete it faster, but still use eco, and off for steep down hills, but use standard to climb the steeper hills faster. Mostly I can do the journey in 30 minutes +or - a minute or two but get plenty of exercise to achieve that. On Thursday I used 40% of my battery (It was very windy) for the 10 miles. Theoretical range going faster using higher assistance modes occasionally 25 miles.
Despite being fit enough to still ride a non assisted I could not do a 24 mile return route to work carrying two loaded panniers without using an ebike. They have made getting to work and back loads of fun and I get a lot of exercise.