One of the best downtown spots to run, no matter the time of day. It's safe, quiet, and uninterrupted by traffic for an almost 2 mile loop.
For urban exploration there is not a more connected trail than the Cultural Trail, whose 8 miles makes a loop in and around the urban core of the city.
Another option for a downtown run, incorporating trendy San Marco Square, an upscale residential district, two bridges, and a section of the Riverwalk Path. Add on or combine with Riverwalk.