Oklahoma???? What does Oklahoma have that Texas does not? Snow-capped peaks? Well, ummmm, nooo...but we have casino gambling!
If you're based in Ft Worth...... the Amtrak train station in downtown FTW is where the once-daily train north to Oklahoma City (The "Heartland Express") originates....but also has trains south to Texas cities, such as San Antonio..... the train is slow, but the station is walking distance (certainly for someone fit like you) to the Riverwalk historic and entertainment area......train might not be much cheaper than flights from DFW, but would save on airport transportation....train schedules at Amtrak.com, and likely more info at Amtrak forum here on FT.....enjoy!