Chief Inspector Dean Haythornthwaite, of Durham Constabulary, said: "We have become increasingly concerned for Jenny since she was reported missing on Tuesday and have been exploring all lines of enquiry, including dozens from members of the public.
Digital intelligence officers have conducted extensive enquiries into Jenny's mobile phone, smart watch and running apps but none have yielded any results.
Emergency teams, including mountain rescue and air support, have covered more than 100 miles of track in the Teesdale area so far with help from the rural community and landowners.
Jenny Hall, 23, was last seen leaving her home in Barracks Farm on Tuesday, February 18 at around 3pm.
"We are focussing our search on areas we know Jenny likes to run through and we are determined to do everything we can to find her.
