More than 300 people are expected to take part in the 43rd annual Ridgeway Challenge.
The Ridgeway is an 85 mile (137km) route popular with walkers
There are three events taking place simultaneously, so competitors can chose between a 40 or 20 mile walk and a 40 mile run.
The Ridgeway is thought to be England's oldest road, running from Overton Hill near Avesbury across Oxfordshire and Berkshire to Streatley-on-Thames.
The event is being organised by the Reading Outdoor Group of the YHA.
Those taking part in the 40 mile events will start at Overton Hill while the 20 mile walkers will start near White Horse Hill close to Uffington.