You can use the standard outdoor Walk activity. When no GPS is available, it will use the accelerometer to detect your steps, and to measure the distance and pace. Though, you have to have already recorded some outdoor walks longer than 2 km under good GPS conditions, to calibrate the watch, before using it indoors, though. Also please note that you cannot hold on the railing or on the handlebars, while walking on the treadmill, because it would prevent the watch to detect the steps. Alternatively you could attach the watch to the ankle.
