The antonyms for the word "runnable" are numerous, and they describe the different degrees of difficulty or impossibility of running. At the opposite end of the spectrum, we find terms like "impassable," "inaccessible," and "obstructed," which indicate that any attempt to run would be futile. In the middle, we have words like "uncertain," "uneven," and "challenging," which denote various degrees of difficulty and uncertainty in running. Finally, at the opposite end from "runnable," we find terms like "uninhibited," "unrestricted," and "free-flowing," which describe conditions that are ideal for running.