You know, Doc, it's gonna be a hell of a long walk back to Hill Valley from here.
3 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:16:46.723 in the DVD version of the movie.
Quote context
[...]
- All right, Doc. Look, as soon as I get there, I'll put them on. I promise.
- Okay. I think we're about ready. I put gas in the tank.
- Your future clothes are packed.
- Just in case, fresh batteries for your walkie-talkies.
- Oh! What about that floating device?
- You know, Doc, it's gonna be a hell of a long walk back to Hill Valley from here.
- You don't want to crash into some tree that once existed in the past.
- This was all completely open country, so you'll have plenty of run out space when you arrive.
- Remember, where you're going, there are no roads.
- There's a small cave over there which will be a perfect place to hide the time vehicle.
[...]