Please show the complete code.
What is the use to redefine each value, such as
let NewHour = newHour
instead of using only newHour ?
Note also that Swift var should begin with lowercase.
If the problem is you need Double, maybe you have to:
let calculatePace = Double(NewTime[0]*3600 + NewTime[1]*60 + NewTime[2]) / Double(3600 * NewDistance)
or
let calculatePace = Double(newHour*3600 + newMinute*60 + newSeconds) / Double(3600 * NewDistance)
This will give hour/km, is it what you want ?
If you want km/h, it should be the reverse:
let calculatePace = Double(3600 * NewDistance) / Double(NewTime[0]*3600 + NewTime[1]*60 + NewTime[2])
or
let calculatePace = Double(3600 * NewDistance) / Double(newHour*3600 + newMinute*60 + newSeconds)
But once again, tell what you expect, what you get, where is eventual crash and show comprehensive code. And don't forget to close the thread on the correct answer once you got it.