What can I expect from 48v 20 amp battery

That would depend on a number of factors. Mainly how much pedal assist you use but also your weight, the bikes weight, tire size etc. I recently tried to figure out how far I could get out of a charge on my bike. I'm mostly flat trail riding and stay in pedal assist 2 . I have a 48V 16 amp battery. After 70 miles I still had 42% battery left. You'll just have to start testing to get an idea of how far you can go based on how you ride. After a full charge, set the trip odometer to zero and go until the battery bars on the display get down to one or two. Check the voltage on the battery to see what's left and compare it to a voltage/% chart for a 48 V battery. I would not run the battery all the way down though. Conventional wisdom says to keep it above 20% to maximize battery life.