How quickly? A leisurely route would be to follow the Thames to Tower Hill, then Cable St (CS something-or-other) to Limehouse. Then the towpath will take you most of the way up R Lee to Royden or Stansted Abbots. The roads from there are much used by Audaxes and fairly friendly through the Haddens, Clavering, under the M11 and into Saffron Walden.