"How long" is definitely time and "how far" is definitely distance, not interchangeable at all for me. I translated this sentence "How long can you run?" and was surprised to see "How far can you run?" listed as an alternative answer because for me they have completely different meanings.
I believe it is 'langt' because it is an adverb.
The adjective 'lang' means 'long', adding 't' changes the adjective to the adverb 'langt' and it now means 'far'.
Noen eksempler fra ordboka mi: 'Lang i ansiktet' (long in the face) meaning dejected, glum. Vs 'Langt borte' meaning far away.