If you're meaning 'Stairway...' you're right, it was never released as a single nor a lot of their other longer tracks. Probably promoted bigger LP sales.
Page did a trick early in their career of deliberately listing an incorrect track length to 'How Many More Times' of 3:30, correct length was 8:28. Knowing radio stations wouldn't play an eight minute song, he wrote the track time shorter to get airplay.
After Stairway, stations didn't have a problem with playing longer tracks. btw, the intro to that song is a direct lift from 'Taurus' by Spirit, whom they often went to see in concert according to the guitarist, Randy California. But, there were a lot of borrowed riffs by Zep. Not an uncommon thing for many bands.
