At first glance, it would seem obvious that it comes from the Norwegian word for "long". They actually have two. "Lenge" means long in terms of time, so possibly it may be used for someone who lived a long time. The other word is "Lengde" which is related to longitude and of course refers to distance.
My wife's from Norway and she is unfamiliar with "Lengh", but I still would suspect that it comes from one of the two aforementioned words.