If you ride the bike path from fry road @ highland knolls past the big parking lot in the middle of the park then northward to where it ends at a dead end road, take the road north over the levee and turn right on the first road after the levee. You will go back onto a path right by the Constable's Annex and can follow that all the way around to where it parallels Hwy 6. Where the path forks near the parking lot along Hwy 6, take the left hand fork and follow the path around and under Hwy 6 and that will put you onto the THP trail and you can ride all the way to BW8 if you want.

Follow the gray line on this map:
http://www.harriscountytx.gov/CmpDoc...GeorgeBush.pdf

FWIW, just riding the GBP path from Fry Road to Hwy 6 @ Briar Forest and back is about 24 miles. The THP portion has more woods and shade, but is not as smooth and is much more crowded with walkers, runners, and folks walking pets. I prefer to stick to the GBP portion of the trail when I am riding just to avoid all the pedestrians.