An endurance bike can be almost anything. Steel bikes are popular with the ultra distance riders as it is comfortable and easy to fix. I have a Genesis Croix de Fer 20. I changed the stock Alex rim wheels for a set of Hunt wheels with a SON 28 dynamo. It is a go anywhere bike and just soaks up the bumps compared to my alu or carbon bike. I can travel light or add a rack and panniers. The biggest difference has been adding aero bars. My average speed has gone from around 17kph to 22kph in the last 6 months since I fitted them.