The verb aruku (歩く) means To walk. The word hashiru (走る) means To run.
I am running = boku wa hashitte imasu (僕は走っています)
100 meter run = 100 meetoru hashi (100メートル走)
The two pictures above are similar except when you use de it means that you are located in that area. When you use the particle o the sentence changes to doing something to something. To understand more look at the differences between these sentences.
I am walking in the mountains
(The location is where something is happening)
I am walking the mountains
(The action is what you are doing in to location)
So the sentence yama de aruite iru shows that the mountain is the location where they are walking. yama o aruite iru shows that walking, is what they are doing to the mountain. Both pictures can mean the same thing, it just depends what you want to emphasize.