There are
807 mi
from Thornton, CO to Seagoville, TX
That's the driving distance. It would take 14 hours 58 mins to go from Thornton, Colorado to Seagoville, Texas.
The flight distance (direct flight from Thornton, CO to Seagoville, TX) is 685.08 mi.
807 mi = 1,298.87 kms