Coming from T2 to T5, walking is by far the most practical. If you have your bags on a cart I would say 20 minutes to do the walk at a comfortable pace. The train is pretty far down, and waiting for the lifts, and the train, can easily make the journey longer. You walk inside all the way, so you are protected from wind and weather.

In Sky City in the middle of the walk there are some decent bars and cafés, if you have to wait for check-in to open. Be warned about Swedish prices in general, and Arlanda prices in specific.