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Fort Worth to Marfa is right around 500 miles. It is pretty much a straight shot west on I-20 to Monahans, then TX-18 south to Fort Stockton, a short jog west on I-10 and then US-67 the rest of the way into Marfa. So you've already got a bit of a mix of Interstates and US highways, but it's a bit heavy on the Interstates. You're probably looking at around 9 hours when you factor in various necessary stops.
As for things to see along the way, there's the Buffalo Gap Historic Village in Abilene, Monahans Sandhills State Park in Monahans, and Fort Davis National Historic Site. Although the last of these is not on your way, it is just north of Marfa in Fort Davis.
AZBuck