20 miles a day is easily doable. Especially if you are riding with plenty of breaks. But if you do a intense 20 miles one day, you might want to take it easy (or even take a break) the next day.

While training, a lot of people make the mistake of not giving their muscles sufficient rest to recover and rebuild. Without which your exercise is doing a lot of harm!

If you are planning to do some specific tour, over the mountains, flats etc., you can then train accordingly, depending on your requirement.