Northies have a nice new place to walk, run, and bike on the newly finished stretch of the Northern Walnut Creek Trail (Phase 1) between Mopac and Walnut Creek Park https://www.yelp.com/biz/walnut-creek-metropolitan-park-austin (near Parmer Lane and North Lamar). The paved path is just lovely, with multiple bridges over creek crossings, plenty of alternating shade and sun, and flora & fauna galore. There are enough gentle hills to give you a workout without being too brutal to keep you from coming back.
If you start on the west side, your journey begins near The Hideout pub and winds about 2.5 miles east all the way to Walnut Creek Park. If you do the out & back course without taking any of the side trails, you'll get a solid 5 mile workout. Of course, if you don't mind the dirt, you can go off the paved path and use many of the rougher technical trails throughout Walnut Creek Park to up your mileage and level of difficulty. The City's map for Phase 1 is here:
https://austintexas.gov/sites/default/files/files/Public_Works/Urban_Trails/2015-05-11_Trail_Overview.pdf and a public MapMyRun route is available at http://www.mapmyrun.com/routes/fullscreen/1122324118/ .
Sometime next year, we should get a couple more miles on the west side of the highway when Phase 1A from Amherst through Balcones District Park to Mopac is complete. For now, that trail remains rough & rocky, but still hike-able; adventurous mountain bikers make be able to use it, but not the casual family rider. The map for that portion is available here: https://austintexas.gov/sites/default/files/files/Public_Works/Urban_Trails/Trail_Public_Mtg_Switchback.pdf
More information about the whole trail system and future development/expansion projects:
https://austintexas.gov/page/walnut-creek-trail-system read more