Most of the "lakes" in the game will have a part that is at least 50 spaces wide. When you start a game if you are near a lake, you should be able to get this achievement. to build a bridge this long it required over 200 logs and about 100 stone for me. Once you get established just save up some resources and put it over the lake. as you select the area you can see how long the bridge will be on the screen. Make sure its at least 50, i did one that was 55 spaces long and it popped for me. Hope this helps, ill answer any questions if you have them.