In this paper, we propose a Cooperative Energy-Efficient Localization Framework (CELF) for power saving in mobile long-thin networks (MLTNs). We use the cycling ad hoc network with fleet cyclists using mobile phones along a common cycling route as an example (our results could be applicable to other scenarios as well, such as the mountaineering party and army troops). As cyclists need to upload their position data and download global fleet information, all power-consuming GPS receivers have to be switched on for obtaining their current locations. In CELF, distributed localization among cyclists without central control is designed to minimize the energy consumption of positioning. To the best of our knowledge, CELF is the first distributed solution for localization which provides the following features: (i) CELF does not rely on costly roadside infrastructures but only employs cyclist-to-cyclist communications; (ii) CELF can supply group members with the up-to-date locations without activating GPS receivers; (iii) CELF can estimate member positions only based on single mobile anchor instead of multiple static anchors. Simulation and experimental results show that CELF outperforms existing works, which can significantly reduce the energy consumption for localization while keeping the positioning accuracy similar to GPS.