The amount of miles you cover on a 20 minute bike ride will depend on the speed you ride the bicycle.
If you ride the bicycle at 3 miles per hour you would cover the 3 miles in 1 hour of bike ride.
So if you ride your bike for 20 minutes at 3 miles per hour then you would cover 1 miles in 20 minutes on the bicycle.
Since 20 times 3 would equal 60 which would be 60 so in 1 hour you'd ride your bike 3 miles and in 20 minutes you'd ride your bike just 1 mile.
If you speed up then you cover more miles in less time so take your miles per hour to figure out the distance depending on the speed.
If I knew what mile per hour you're actually planning to ride your bike then I could give a more accurate answer.