San Antonio can be a challenging place to do elevation training. However, one local park has 4.5 miles of trail loops with the outer parameter taking about 35 minutes to jog and depending on your tolerance for going off the marked trails, has several steep inclines especially in the wash outs. I am talking about Comanche Lookout Park.
The parking lot sits at approximately 950 feet in elevation and the site is listed as the highest point in Bexar County with an elevation of 1,340 feet. Which, I can not confirm on the available topographic maps of this site as they indicate the site peak is 1,034 feet. Regardless of real elevation, you can do elevation training here as there are steep grades and you can complete multiple loops to simulate higher or longer climbs.
With the deep natural surroundings at this park, it is hard to believe you are next to a major road. Making this park a great retreat from city streets. Also, if you are in good condition, load up your daypack with about 20 pounds of gear and jog these trails for a serious workout!