Forget it. Just forget about it!
The mile chasing, that is.

Miles are just a curiosity. It's more about time. Go for a 15 minute ride. Or go for a 2 hour ride. I use a Garmin Edge 500 and like to chase my average speed for motivation, but I don't go out to "ride 20 miles". I go out to ride for an hour and a half.
The reason is because my 3 miles are going to be a lot different then yours or the next person's. Unless you're looking for guidance from someone who rides the same terrain and under the same wind conditions as you, miles mean nothing. Certainly your body has no idea how many miles, but it will respond to how many minutes of exercise you perform.
If you want to work up to being able to ride an Imperial Century (100 miles, not kilometers), plan on being able to ride for 7-8 hours.