In Ledecky's case the clock is the only competition, and she's just so much better than anyone else it's currently impossible for someone to race her.

In Athletics at non-Championship meets they have pacemakers who provide a pace for the eventual winners to chase when they are going for fast times.

It'd be interesting to see what Ledecky could do if she swam (for example) with male pacemakers as competition.

Or maybe Summer Sanders will move up to the 1500m and we'll see a real race before Ledecky retires.