The walking distance is about 100 km. The only major obstacle is the shortage of drinking water en route. The hikers have to carry their water supplies with them after all snow patches have disappeared in summer.
Day 1 The first leg between the huts at Herdubreidarlindir and Braedrafell is 17-19 km long. 
Day 2 The next leg to the Drekagil hut is 18-20 km long. 
Day 3 The third leg across the Dyngjufjoll massif via the Drekagil gorge, Viti and Oskjuvatn and eventually through the Jon“s Pass to the hut in the Dyngjufjoll Valley is 19-20 km long.
Day 4 The last leg covers 22-24 km of lava and sand deserts until the small oasis Sudurarbotnar is reached. There are a few excellent books available on hiking routes in Iceland.

Huts on hiking trail Herdubreidarlindir to Svartįrkot

Herdubreidarlindir hut

Braedrafell hut

Dreki huts

Dyngjufell hut

Botni hut

Click the linked parts of the map for further information!