From the beach of Aghios Dimitrios on the east side, take a small goat path that runs parallel to the sea. A small beach at Strovili leads to the bottom of the Gorge.
From this point, take a rugged 4 km natural gorge leading up to the area of Dhiaselo, which is close to the new reservoir. The walk up itself takes a bit of clambering over some boulders but this should not cause an issue for the fit walker.
From the summit you can choose to end the walk at the top of the Gorge and transport can collect you from there, or alternatively you can make a circular walk back down footpath 11 while enjoying stunning views onto Peristera, the Marine Park and surrounding islands. This will bring you back close to the starting point at Aghios Dimitrios peninsular.
Alternatively, you can start the walk in the opposite direction, going down the Gorge, but many find it easier on the knees to walk it up.
This is a brief description of this popular walk, a walkers map and walkers book would be a useful accompaniment.