Attached is basically the full route you can do on GBP and THP. I usually do GBP from one of the lots on HWY 6 all the way out to where Rudy's is on I-10 frontage road (the end of the Mason Creek greenway. I have to cross Fry Rd at the end of GBP and go through the little park they have there before it hooks up with the Mason Creek path, but I'm sure you've done all that.
I did this route on new year's day, came in at a total of 47.3 miles.
The easiest, well, really, only, way to get to THP from GBP is to hang a left off the main straight path that parallels HWY 6 at the spillway, and follow it. It will take you right up to the HWY 6 bridge, cross it, then loop back around underneath and bam, you're in THP. Follow it all the way to the beltway if you like - which is what I did on the attached map. I also did the extension that you can get to from the parking lot on Memorial - that's what the little hook is at the middle top of the route.
I'm assuming that means you know that when you reach the gates at the north end of the long straightaway through the center of GBP, that you just stay on the road, go past the apartment complexes on the right, and hang a right onto the access road that leads you right to the northern stretch of GBP that parallels I-10.
txtags is right though, THP has a tendency to be jammed on nicer weekends. During the day on a weekday though it's a breeze. However the condition of the path leaves much to be desired. Watch out for cracks, crevasses, and ripples from roots that would knock the wheels of a Semi. There's an especially terrible stretch of it between Kirkland and Wilcrest, with an especially terrible part right before Wilcrest (heading east), and when you're coming back just after you've come up the hill after going under Wilcrest and you're rolling down...BAM into freaking death roots.
Hope this helps.
Maybe I'll see you out there!
PS - I love riding the Mason Creek path from Fry Rd to I-10 frontage because it's SO SMOOTH! (and all the racing roadies stop when they get to Fry Rd and then turn around and go back to GBP, so it's so wide open!)
Last edited by the sci guy; 01-07-14 at 10:59 PM.