iOS Swift 4 Tutorial on Workout Extensions

First of all, I have read through some of "similar" enquiry, but none seem to specifically describe the issue I encountered. Most are Swift 3 and earlier, which I am currently unable to comprehend. Please pardon my small request to sought for some help here.

struct Workout {

     var distance: Double

     var time: Double

     var averageHR: Int

}

extension Workout: CustomStringConvertible {

     var description: String {

     return "\(distance) metres at \(time) seconds with HR \(averageHR)"

     }

}

extension Workout {

     static var speed: Double {

          return distance / time

     }

     func harderWorkout() -> Workout {

          let newDist = distance * 2

          let newTime = time * 2

          let newAverageHR = averageHR + 40

          return Workout(distance: newDist, time: newTime, averageHR: newAverageHR)

     }

}

The question asked in the tutorial is "Now create another extension for Workout and add a property speed of type Double. It should be a computed property that returns the average meters per second traveled during the workout."

And that's where I'm stuck, to specifically go according to the instructions given. The best I can do is to create a workaround using a static func instead

static func calculateSpeed(distance: Double, time: Double) -> Double {

     var speed: Double { return distance / time }

     return speed

}

But it seem forced. Would anyone know what would be the "correct" way to actually solve this? I think is more of how to compute the properties within the structure, which I am unable to recall or search for any answers. I really appreciate any help rendered.