Ah, μακριά is the feminine version of μακρύς? Well then, in the sentence above the noun is masculine- "δρόμος," but what if it were "οδός"? Would the sentence then be "Αυτή η οδός είναι μακρύτερα."? And then that would be ambiguous, no? (i.e. meaning both "This street is longer" AND "This street is farther"?) Does it matter if the adjective is predicative or attributive? "Η Νέα Ζηλανδία είναι μία μακριά χώρα." Can this sentence be both "New Zealand is a far away country" AND "New Zealand is a long country"? (both of which are true).
In your first example, longer would be either μακρύτερη or μακρυτέρα (which is a more ancient like spelling) so there would be no problem. In you second example, μια μακριά χώρα is always a long country because the word μακριά is next to a noun, so it's an adjective. A country farther away would be μια χώρα μακριά απο εδώ= Far away from here. Or μια μακρινή χώρα= A far away country (because far away here acts like an adjective,you use the adjective μακρινός). If you said Η Νέα Ζηλανδία είναι μακριά, then there would be a problem if you mean long (as an adjective) or far away (as an adverb), but you get the meaning from context and the speaker (who knows that can be misunderstood) uses another word/explains what he says (p.e he could say μακριά απο εδώ as in the above example=far from here and problem solved!)
Many things can be longer, but (to simplify things) only one thing can be the longest of all.
So "the longest" is definite.
μακρύτερος is "longer" but ο μακρύτερος (with the definite article) is "the longest".
Similarly with other comparatives: they turn into superlatives when you put the definite article in front of them.
Εγώ είμαι καλός, εσύ είσαι καλύτερος, αλλά αυτός είναι ο καλύτερος. "I am good, you are better, but he is the best."