For real miles you have to get out on the streets, preferably out of town with people. That means you have to do FWBA rides. That's the good news.
People keep telling me that you can take the Trinity trail system to Weatherford and then on to Mineral Wells (!). Wow....I haven't done that. It is supposed to be a crushed cinder surface trail, but you know, I really am a roadie....the speeds I do are not really suitable for a multi-use trail system.
If you can get over to Joe Pool Lake dam, you can do laps back and forth on top of the dam, 8 miles per lap, between the gates that keep the cars out, allowing only non-motorized use on a road-width surface.
If you think you are slow, you can catch one of the FWBA "Not Just for Beginners" rides (see ride schedule on website!), out of Joe Pool. Seems to me they average about 14mph and stop a lot. Or you could ride with Lone Star Cyclist's Club. They have a mild pace.
Yes, okay, everybody has to start at the bottom. But you can work your way up to longer and faster rides to the degree that you desire. You are going to end up on the road if the trail system is not serving you well.