Texas Motor Speedway is actually about 20 miles north of downtown Fort Worth, near Alliance Airport (a general aviation airport).

If you are set on a FS property, your best bets will be the Renaissance Worthington in downtown Fort Worth (I've stayed there - good property), the Marriott Solana in Westlake (never stayed, but I know Randy Petersen likes it), or perhaps one of the DFW properties.

The Worthington is around 20 miles away, the DFW properties are about the same, and Westlake will be 10 or so. From DFW or Westlake you will need to take 114, which I understand is pretty much a parking lot on race days. From Fort Worth you take I-35W, which is also a parking lot north of I-820 on race days, but apparently not as bad as 114.

There's a Residence Inn by Alliance which is a relative hop, skip & a jump compared to the other options.

A non-Marriott option might be the Doral Tesoro.