Distance:
Fitness trail: 0.5 miles (not quite, according to my Garmin)
Nature trail: about 3 miles round-trip
Walking: yes
Running: yes
Biking: yes; mountain (good for beginners)
Dogs: yes
Kid-friendly: yes
Wheelchair/stroller accessible: yes
http://www.charlottesville.org/index.aspx?page=384
Pen Park is a great place to spend the afternoon. It has tennis courts, a baseball field, picnic shelters, a playground and fitness and nature trails.
To get to the trails, turn left just before the tennis courts and follow the road down to the parking area. The trailhead is located behind the picnic shelters.
The fitness trail is paved, and there are several exercise stations along the way.
The nature trail is wide and sandy. There is a fairly significant climb at the beginning, but then it levels out. 
The playground is excellent, with separate sections for younger and older kids. The spongy surface is great for keeping kids safe, and also perfect for doing a few strides or high skips!
To get to Pen Park from Route 29, take East Rio Road to the stoplight at the intersection of Pen Park Drive. Turn left.
From town, take Park Street to the stoplight at the intersection of Pen Park Drive. Turn right.
Nemophilist in Cville 