Problems with prepositions

And one day when an old lady with rheumastism came to seed the Doctor, she sat on the hedgehog who was sleeping on the sofa and never came to see him any more, but drove every Saturday all the way to Osenthorpe, another town ten miles off, to see a different doctor.
And my question, why in the text is used miles off? Does it mean the same what miles away? I mean if in this context off means away with the same meaning, if not what is difference?

yes, it means how far away it is

Is any rule to ensure when to use away and when to use off or do I have to have better "sense of English" to know it?

Mamy takie zdania:
1. he is away on his holidays
2. he is off on his holidays
Ja tu widze duza roznice, a Ty?

Maybe I surprise you but I always had problems with prepositions and to be honest I have no idea what is difference. :) Could you explain me it?

When you 'feel' the language - you will know.
1. on juz tam dawno jest
2. dopiero co jedzie...

So when you are away you somewhere for longer time and when you are off you are getting to destination, right or not?
And what about with my first problem that is: a certain person rides to doctor who is milles away/off - do you see any difference?

miles... i ....rides to THE doctor,
W tym akurat, ja uzylabym 'away.'
Jest tez...we're off to see the Wizard, the wonderful Wizard of Oz,