I used to find it difficult to just ride for the sake of racking up miles.
Here are a couple of places I've ridden and been suprised by my total miles at days end:
*Hamlin Beach State Park....pack a picnic lunch, take your bikes along and make a day of it. A complete circuit of the park including all the camping loops is seven miles. You'll be so busy people watching that the miles will just fly by.
*Holy Sepulchre Cemetery is dead flat (excuse the pun) between Dewey and Lake. It's teaming with wildlife and beautiful sculptures. If you want a little challange you can cross Lake Ave. and ride down to the cemetery's three ponds. After you're stronger you can pick up the Charlotte section of the Genesee River Trail right at the north edge of the cemetery. From the trailhead at Lake Ave to the north end of the trail at Pelican's Nest is three miles.
Every inch of these locations I've listed is either blacktop or concrete riding surface.