From downtown, running along Allen Parkway and then circling at Shepherd, and coming back up Memorial Drive is really nice. I think that loop is probably 6 miles. For a more difficult run, there are trails through the Allen Parkway park area. I would only recommend those during the daylight. If you want to extend the run, take Allen Parkway, pass Shepherd, and cut over to Memorial Drive and run to Memorial Park. For a really long run, take a loop around the park. It's 2.9 miles. Or just keep running down Memorial Drive. After you cross under Interstate 610, the homes and neighborhoods are beautiful. But the whole run is nice. That's a really pretty part of Houston.