The Plano Bicycle Association rides or the Greater Dallas Bicyclists will probably have saturday and/or sunday rides in the lengths that you are looking for. I know the PBA's Saturday morning Distance Builder 1 group usually does 60-ish miles at an average of around 17-18. I rode with the DB2 group last weekend and we did 52mi with an average of around 17.5. There are faster and slower groups from there.