Objective criteria are something that is definitively supported by data, and not open to interpretation. To keep this on touring, saying the Katy Trail is 240 miles is objective. Saying that it is short, long, easy, hard, etc. is subjective.

Going back to your criteria, you could objectively say that it is shorter than a median touring day (assuming you had the data to show the median was higher). I can objectively say that it is longer than the average distance someone on a bike rides in general (assuming I had the data, which I don't, but like you I feel quite comfortable making the assumption). Both of can subjectively say that it is long or short based on either of those criteria. That is why it is subjective.

I can't say that the Katy Trail is not 240 miles, because it is. That is simply a fact. What is short, hard, long, or any other sort of descriptive word you can come up with to describe riding on it is open to interpretation, ergo not objective.