A decent spot to get in some flat off-road running is to head to the 700 Years Stadium and Sports Complex just north of town. The stadium itself has an excellent running track inside. Just outside the stadium, there are numerous sports and recreational facilities, where runners can do some good flat laps on a road around part of the stadium and a couple of long flat sections that are great for interval training. This is a particularly popular place to come at night, as it’s well lit and off road. In addition to the running track, one could do ~2 km around the complex itself.
If you’re into running tracks, another good spot is at Central Stadium, located on the campus of Chiang Mai University.