Canyon Loop Trail is excellent and very runnable. Take 22nd east to Spanish Trail, follow signs to park. Rillito River Park - Flat, bike path 9 mile out and back run starting from River and Campbell Boulevard running west to La Cholla Road. Asphalt bike path with dirt trail on both sides of path. No traffic, only bikes and skates.
Path goes under all major streets.
Lots of great trails starting from the Equestrian Center or from the Trailhead. Map available at Visitors Center. Canyon Loop and 50 Year Trail are the best. Some trails a bit sandy as they are also used for horse trails. In Northwest Tucson, take Oracle Road north past Oro Valley, Park is on the right just beyond the huge Allied Signal complex.