Since that's probably going to be a rarity unless you are on a sheltered rail-trail, it's probably not the best question to ask.

In any event, 20 kms each hour with 30 min. breaks in between comes to 7 hrs. on the road. IMO, that's a long time for only 60 miles of primarily flat, windless riding. Add in stops for photos, map checks or just to admire views and you stretch the day out ever more. Also, it doesn't often break down so neatly. You may go for a long stretch with no services, necessitating additional stops in places whether there is food and water.