Howdy. I am in Greenville, but frequent Dallas and Farmers Branch for mtb riding.
I know of a trail we can do on mbs if we pick a time that it is not crowded. It is a jeep track for several miles from US-380 Farmersville, and later turns into a pretty flat, crushed limestone singletrack that runs up to Lake Ray Roberts State Park. It is very easy to navigate, and does not have close trees or sharp turns. I do not know if motor bicycles are excepted from the "no motor vehicles" rule, but I expect that no one will bother us or report us if we are polite and do not "buzz" anybody. The weeder engine bikes seem much more "socially acceptable" to many path users in my experience, esp at a low cruise instead of full roar. Roadies generally won't ride with me though or even wave (bike snobs).
Ray Roberts Lake State Park - Greenbelt | DORBA