There are two main routes to come into Big Bend from San Antonio. I-10 takes a northerly route to Fort Stockton. You may also take US 90 through Uvalde and Del Rio, which is a little shorter distance wise, but takes a bit longer.
From Fort Stockton, you can go south on US 385 to Marathon, 58 mi. then continue to Big Bend National Park east entrance 30 mi. further. Park Headquarters at Panther Junction is another 40 mi. You can also continue on I-10 for 9 mi. , then turn south (left) on US 67 to Alpine, another 58 mi. This is a more roundabout way to get to the Park, but closer to Terlingua, Lajitas, Ross Maxwell Scenic Drive and El Camino del Rio. Enter Alpine, right after you pass Sul Ross State University, the road divides. Go about two blocks take a left on TX 118. Go 78 miles to Terlingua/Study Butte Junction. Take a right on TX 170 for Terlingua, Lajitas and Big Bend Ranch State Park. Keep going straight on TX 118 3 miles to Big Bend National Park entrance.
The other main route from San Antonio is to take US 90 through Del Rio into Marathon. From Marathon, either head south on US 385 as above or continue 30 mi. into Alpine. This southern route is shorter and more interesting, but may take longer due to more traffic and several small towns. I-10 can be quicker; after you pass Kerrville the speed limit is 80 mph.
By taking different routes to and from Big Bend, you can see lots of Texas.