I used to run there and back on a 3.5 kilometers long road,

[tufguy]

Some time ago I used to go running. I used to run there and back on a 3.5 kilometers long road, which made a total of 7 kilometres.


I read "I was 20 miles there and back" on a dictionary. Does it mean 20 miles one side or both?

Are my sentences correct now?

 

[Raymott]

Do you mean "It was 20 miles there and back"? That is the round trip - the total distance.
However be aware that "I used to run there and back on a 3.5 kilometers long road" does not mean you ran 7 km, because you haven't said where you ran to and from. You've only given the length of the road you ran on.

 

[SoothingDave]

It's "3.5-kilometer long." We don't use the plural when using a measurement as an adjective. A 6-inch sub. A 10-foot pole. A 55-gallon drum.