B777 Pilot reported failure to remember that no tug was attached on engine start resulted in an uncommanded roll forward and a loss of control event.

Narrative

The aircraft was located at a handstand. Took the bus to the aircraft; boarded and began aircraft preflight duties. Once aircraft doors were closed; informed ground personnel we were ready to go. He said that I was clear to release the brakes. As I did that; the fo informed me that clearance had given us a wheels up time of an hour later. I informed the ground guy and we sat there at the handstand for an hour. Once clearance had informed us we were clear to start; I once again informed the ground guy and he said that we were clear to start. Important note: this hard stand was one that you can just taxi straight out of. I had never been on one of these and it didn't really register initially in my head that we had no tug attached. When he cleared us to start; I then realized that was how this worked and I told the fo he could start the engines. Just as the engines were started; the ground guy starting telling us to stop; stop; stop. I wasn't sure what he meant and told the fo to cut the engines off. It was at that time that I realized we were rolling forward and immediately hit the brakes. We had only rolled a few feet. No one was injured and nothing was hit. Cause: Assumptions and routine were the causal factors here. The ground personnel routinely clear us to release brakes when we are ready for departure; and we do the same. Neither of us really gave any thought to the fact that this was a special circumstance that required the brakes to remain set. I; initially thinking that we might be pushed; and not correctly assessing the uniqueness of this particular hard stand; was easily drawn into what could have been a dire circumstance. Thank God nothing happened. Only a lesson learned.