Distances travelled over two days in former time (6,4)
I believe the answer is:
'former time' is the definition.
(period in history)
'distances travelled over two days' is the wordplay.
'distances travelled' becomes 'mileages'.
'over' means one lot of letters goes inside another.
'two' says to double the letters.
'days' becomes 'd' (abbreviation for days).
'd' duplicated is 'dd'.
'mileages' enclosing 'dd' is 'MIDDLE AGES'.
'in' acts as a link.
(Other definitions for middle ages that I've seen before include "European history from about 500 to 1500 AD" , "former times" , "historic period" , "some time in the past" , "way back" .)