A small correction to an earlier post. It's been stated that the pilot was used in the first five episodes, but really it was only used in four of those five. The second episode, "The Derelict", didn't use much footage from the pilot except for short inserts of the Jupiter 2 flying through space, and a quick shot of John Robinson writing in his diary with new narration.
The other four episodes, 1, 3, 4, and 5 all used big chunks of the pilot as their story, while #2 was more of a "new" episode detailing some of the Robinson's adventures in the hostile environment of outer space.
The diary shot in The Derelict wasn't from the pilot, actually. John is in his spacesuit in The Derelict. Once they landed in the pilot, they only wore their fatigues. So the only pilot footage in the episode are the quick space shots and the Jupiter 2 approaching the planet (which itself was changed).