Chief Inspector Dean Haythornthwaite said in a statement: "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 also carried out extensive enquiries into Ms Hall's mobile phone, smart watch and running apps, but the force said "none have yielded any results unfortunately".
Sniffer dogs are being used in the area, with the search concentrated on running trails between Eggleston and Hamsterley used regularly by Ms Hall.
Jenny Hall, 23, was last seen leaving her home in Barracks Farm, Tow Law, County Durham, in her car just after 3pm on Tuesday.
A major police search is taking place for a runner who has been missing for three days - as officers say they are becoming "increasingly concerned".
