Yeah, an end would make sense too! I was imagining a timer (e.g. 2 minutes) for you to build as far as possible, and then it restarts.
The thing that felt a bit uncomfortable was the "endless" style, which kind of progressively reset your progress and made it less motivational to get back.
But those are just some game design nitpicks! Overall the mechanic was very fun to play with and I'd love to see it used in a bigger game. Definitely gonna try out Box2D in my spare time.