Appalachian Trail
http://dnr.maryland.gov/publiclands/at.asp

Inner Harbor Loop 
(aka "Balti Waterfront Promenade")
http://www.waterfrontpartnership.org/promenade_home.aspx

Cylburn
http://www.cylburnassociation.org

Ruxton Road/ Robert E. Lee Park

Loch Raven Reservoir: Merryman Trail
http://www.trails.com/tcatalog_trail.aspx?trailid=XMR012-025

Gunpowder State Park
http://www.dnr.state.md.us/publiclands/central/gunpowder.asp

Gwynns Falls Trail
http://www.gwynnsfallstrail.org

Oregon Ridge
http://www.baltimorecountymd.gov/Agencies/recreation/programdivision/naturearea/oregonridgelodge/

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For More Info: 

http://baltimore.cbslocal.com/top-lists/best-walkingbiking-trails-in-maryland/

http://www.americantrails.org/resources/statetrails/MDstate.html