Adder2 'hard to bear' does not mean 'hard'. You've changed the meaning of the word by attaching it to a different concept
Yes, I understand what you're saying. I get that, in Spanish, this is a way of saying "this is a long distance". It's still an akward translation in English, though. "The distance is hard" is just not something that would commonly be said because there are better ways to say it, i.e. "the distance is long".
It isn't the distance that is difficult/hard. In that context, the difficulties are numerous, and they are caused by the distance, or by the seperation that is caused by the distance, but the effects of a thing are distinct from that thing.
It's like saying "John knocked Jose down," when in actuality Tina pushed John into Jose. Technically, John's body did cause Jose to lose balance, but without Tina having pushed him, it wouldn't have happened. Don't blame the distance. People choose relationships, Tina chose to push John; the concept of distance itself did not make a decision on its own attributes.
That was a really longwinded way of saying "context matters..."
It's a common phrase in the Midwestern United States. It wouldn't have to be a "long" distance to be hard or difficult. It could mean a difficult hike or climb. Yes, the distance is difficult would be a correct response also. However, in my area, the distance is hard would be easily understood.
"The distance is hard" in English (meaning difficult, as in a long-distance relationship or a tough running trail) seems like slang, not correct grammar. "The Spanish test is hard," makes more sense, because you are describing the test itself. In the case of distance, it's not the distance itself that is difficult, but maintaining the relationship, or running so far. "Running that distance is hard" or "having my significant other living in another state is hard" makes sense, but "the distance is hard" is a form of verbal shorthand. I might say it in conversation, but would not use it in formal writing, because it's not actually correct.