There are several trails that are great for running
a few you can look up are White rock lake (9.3 miles around) - I would only run this with a buddy as there are some lonely parts on this trail and it is just safer. That said this is one of my faviorte trails to run - it has an even mix of good hills and flat running.
A little north in Plano you have the Chilsom trail (3+ miles) and Bluebonnet (I have not measured to the end of this trail yet but I know it is over 4 miles out) trails
Hope this helps - White Rock Lake is part of the Dallas marathon course in case that is of interest to you and is also closest to Downtown Dallas