Colorado Bend State Park
Colorado Bend State Park, located about 30 miles from Lampasas, Texas on the Upper Colorado River, has over 20 miles of rideable trails. Like most state parks, however, the majority of the trails in Colorado Bend weren't designed specifically as mountain biking trails, so the flow is not generally great.
The official trail map is more of an impressionist artistic rendering than an actual map. If you squint hard enough, you can make it resolve into an abstract shape vaguely resembling the general layout of the trail system, but there are a lot of marked trails which the map blissfully ignores. The trails in Colorado Bend are marked by either green, yellow, orange, or red posts, although what these colors represent is anyone's guess (they do not correspond to difficulty, for sure.) Some of the trail junctions contain signage, but most trails contain only these enigmatic colored posts, leaving you to guess which trail you're actually riding at any given moment.
The best trails for intermediate level mountain biking are Lemon Ridge Pass and the Cedar Chopper Loop (and its tributaries.) Technical obstacles on these trails consist mostly of rock gardens, babyheads, and gunsights. Because the trails are still relatively young, the rocks are somewhat sharp (Pinch Flat City), and the lines through some of them are less than clear. The frequency and length of the rock gardens will likely be frustrating for beginners (see above comment regarding lack of flow.)
Starting from the trailhead at the northern end of the camping loop, ride the non-technical, flat, scenic River Trail for 0.5 miles. Veer left at the trail split (this split is not shown on the map) to avoid going to the River Backpack Camping Area. At the major trail junction (Lemon Ridge Pass), continue straight. At about mile 1.5, be extremely cautious of the washed out culvert crossing. The creek apparently washed this out during the 2007 floods, and it hasn't been back-filled yet. The drop to the right is about 15-20 feet, easily enough to break a bone. TPWD has placed a small slab of limestone over the gap for hikers to use, but it isn't wide enough to ride safely. Speaking from first-hand experience, if you catch your front tire on the transition, you will likely fall off into the abyss. If you're lucky, the pipe will break your fall.
At Mile 2.3, follow the River Trail left and up a series of moderately technical and moderately steep switchbacks to the top of the ridge. The River Trail then becomes a very scenic rim trail for a short distance before hitting the Cedar Chopper Loop. Turn left and ride the longest part of Cedar Chopper, a flat-to-rolling intermediate level trail with lots of rock gardens. Veering left at the next junction onto Old Gorman Road, you are treated to a 1-mile-long bomber downhill, possibly the longest sustained technical descent in Central Texas.
Old Gorman Road dumps you out at the Gorman Falls area. Follow the trail marker left, past the ranch house, and over the wooden creek bridge to the maintenance road. Ride up this road a short distance and look for a bike rack on the right. This is the trail down to Gorman Falls. The trail is not rideable, but if you haven't seen the falls, the short (1/2 mile round-trip) hike down and back up is highly recommended.
From here, I recommend riding back up the Old Gorman Road the way you came in, then right on the Cedar Chopper Loop, then exit the Cedar Chopper Loop at the parking area. Take the main park road back east toward the campground and watch for the Lemon Ridge Pass trailhead on the left. The Lemon Ridge Pass trail starts off as a doubletrack, but the singletrack cuts off to the right almost immediately. This section of singletrack appears new and needs to be broken in. The new singletrack dumps you at another trailhead. Bear left here onto the old singletrack, which provides a rock-garden-filled rolling, twisting ride over the ridge and then a fast, moderately technical downhill back to the River Trail. Turn right on the River Trail and ride back to your car. Total distance for this loop is about 12-13 miles.
For beginners, the trails to the southwest of the road (Lively Loop, Windmill, etc.) are generally double-track ranch roads which roll through the meadows and cedar breaks along the park boundary. These are mostly non-technical, but they also can become overgrown with nettles and other mean flora in the spring and summer months. These plants don't feel particularly pleasant brushing against your shins at 10 MPH. Also, the trails in this area of the park have no signage whatsoever, and some of the junctions are not on the map, which makes it very easy to get lost. Bring a GPS.
The Tie Slide Trail, at the far upper end of the park, is accessible by riding the maintenance road up from the Gorman Falls area. Tie Slide is scenic and actually has some flow to it, but it is unfortunately very underused and has become overgrown and almost impossible to find in places. The Gorman Falls Trail is the most technical trail in the whole park and consists of almost 1/2 mile of continuous (and difficult) rock gardens. However, this is also the main hiking trail down to the falls, so while bikes are technically allowed on it, it is virtually impossible to get sufficient speed to clean the obstacles because of the heavy traffic. These hikers do not understand mountain biking, in general, and many of them will look at you as if you just deuced on their souffle'.
The main 6-mile-long park road is unpaved and hilly and can be ridden purely for distance. The scenery along the road mostly consists of cedars and more cedars, however.