Assuming you walk 4 miles per hour which is a pretty average walking speed you would cover 20 miles in 5 hours if you didn't stop for breaks.
However you would stop for breaks on the 20 mile walk and likely rest for 10 to 20 minutes at a time so factoring those breaks in you could expect it to take 7 to 10 hours to walk 20 miles.
Of course if you walk a bit slower then it would take you longer but the faster you walk the faster you would cover the 20 miles.
But if you plan to walk 20 miles then plan for it to take all day but you should be able to cover the 20 miles in 8 to 10 hours.
You'll get a lot of exercise and could lose a lot of weight when walking the 20 miles in a day but you'll be wore out for the day after walking 20 miles.