There are some OK trails around here, like the one at Grapevine Lake and the City of Corinth has put in 19 miles of trails but nothing really challenging, especially compared to some of the trails I used to ride in Canada. Here's one of them : Downhill Freeride MTB - North Vancouver - YouTube I could handle the real bad-ass trails 30 years ago but now I find the ones around here all I can handle.
As for the topic at hand, I used to go mountain biking when I lived in Central Commiefornia and Arizona. I haven't gone in over ten years. I bet my legs would be like mashed potatoes by the time I'm only a quarter of the way UP a grade.