The Denver metro area has more than a hundred miles of walking, jogging and biking trails, and many more choices in the city-owned parks in the foothills outside of Denver. You’ll find list and maps at Recreational Trails. On it you will also find links to trails in the surrounding cities.
If you are a bicyclist, this map outlines bike routes and trails. The city’s B-Cycle program, primarily in the downtown area, allows anyone to sign out a bike at one stand, use it then return it to that stand or another in the city. Fees are very reasonable. You can purchase an access pass with a credit or debit card at the B-stations.