You could create a table with 3 columns: distance, time, pace. Set the display format for time and pace to "Duration".

Enter these formulas:

  distance = time / pace
  time = distance * pace
  pace = time / distance

Drag fill everything down. At this point you get reference errors, but once you enter any two values (thereby overwriting the formulas in those cells), you get your result.

You just need two spreadsheet tabs. One for the "raw" input and one with a formula that either takes the input if it exists or falls back to the calculated version

Came here to see if anyone mentioned propagators. That thesis is excellent. I second the recommendation.