I'm part of the Memphis REI team. I absolutely LOVE biking in Memphis, whether I'm on a cruisey ride on my MTB, or a longer ride on my road bike. I know you posted this six months ago, but have you had a chance to do many routes? What all have you done?
There are two large road bike groups in Memphis: the Memphis Hightailers, and the Memphis chapter of the Major Taylors. Pre-COVID, both of these groups did very regular group rides. There are membership fees for the rides, but it's totally worth it.
You can go from downtown to Shelby Farms and past that into Germantown along the greenway, and doubling back will get you at least a 30-40 mile ride, depending on which streets you want to ride on in Germantown. You can also ride out to Shelby Forest from downtown Memphis; I would suggest doing this on a weekday so traffic isn't as bad, and definitely carry and extra two tubes because of the potential of flats, but that will get you a 40-miler in as well.
My usual route is to start at Memphis Made, head north on Central to Peabody, take Peabody to downtown, ride over to Arkansas across the bridge, tool around downtown for a bit, and then take Madison back to Midtown, then hit Central back to Memphis Made. That's a super easy 12-15 mile ride.
Feel free to DM me, and I'm more than willing to share my Strava routes with you to give you a more concrete idea of where to ride to rack your miles up! 🙂