Sam Houston National Forest is a bit less than an hour north of
downtown Houston; take I-45 to FM1375, go west on 1375 for a couple of
miles to Forest Road 233 and follow the signs to the gravel road that
leads to the trail parking area for the East Unit; there's another unit
on the other side of the lake, but I haven't ridden those yet. These
are "multi-use trails"; dirt bikes and ATVs are explicitly permitted.
There's a $5 per-user-per-day fee required.

There are quite a few miles of trail in this section; I didn't find a
figure in the USFS information, but I'd guess that there are around 25
to 30 miles of total trail in the various loops. None of the loops
appear shorter than 5 or 6 miles, and some are considerably longer.
It's all through gently rolling pine woods, with some roots to bump
over but very few rocks. Mud is a hazard if it's been raining
recently, which has been the case this year for most of the summer.

Map he http://www.fs.fed.us/r8/texas/maps/sam_mu_east1.pdf

Unfortunately, somebody seems to have decided that it would be a spiffy
idea to "improve" the trails by laying down a couple of inches of sand
on them. Not sand and pea gravel, not sand and clay, but just sand.
If it's wet or at least thoroughly damp (which most of it was) and
hasn't been chewed up by dirt bikes (which, alas, too much of it had
been), it's not a bad riding surface. If it's dead dry and/or has been
fluffed by motorized traffic, it's wheel-trap city; expect to get out
of shape easily, often, and badly. I was there on a Wednesday
afternoon; the trails were utterly deserted and we had them all to
ourselves...but there were many more ATV and dirt bike tracks than mtb
tracks visible, and I suspect that weekend use can be a bit dicey at
times.

Grades are mostly short and shallow on these trails; not much technical
stuff, but enough to keep it from being just a speed run. Wildlife
sighting count (other than bugs) was zero, though there were lots of
deer tracks. No mosquitoes, no gnats, nothing nasty and biting at all
for that matter.

I'll go back sometime if I have a weekday free, but I'll check the
trail conditions before I drop another fee envelope in the box; if the
sand hasn't been pounded into the clay enough by then, I might give
this a miss and head somewhere else to ride.