approaching vs run-up

approaching and run-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
approaching	Yes	Yes	No	No
run-up	Yes	No	No	No

As nouns, run-up is a hyponym of approaching; that is, run-up is a word with a more specific, narrower meaning than approaching:

Other hyponyms of approaching include access, closing, closure, landing approach.

approaching (noun)	run-up (noun)
the act of drawing spatially closer to something	the approach run during which an athlete gathers speed
the temporal property of becoming nearer in time	a substantial increase over a relatively short period of time
the event of one object coming closer to another

approaching (adjective)	run-up (adjective)
of the relatively near future