Painting is the artist's attempt to communicate a personal impression about a subject. If you love the landscape, then like myself, you probably stop in your tracks daily to study natural phenomena; a rising thunderhead or the way light plays in and out of the trees along a shady trail. Funny how a snap-shot of the same scene tends to disappoint us. Why?
Because our eyes focus on one thing at a time and our brain says "Wow. Look at that", while everything else fades in importance. The camera, on the other hand, reduces the whole scene to the same level of importance. The very thing that inspired us to shoot the photo is lost in all the other elements of the landscape.
So, what's your BIG idea?
Simple ideas, communicated in the most simple way, are the most powerful: that really big tree, a boat on the river, the derelict lounge chair in front of an old cabin.
Take that kernel of inspiration and see if you can fit it to one of the compositional themes previously discussed. A sound compositional idea is the very thing that will hold your painting together and keep you from making arbitrary choices that confuse the viewer and detract from the big idea.
Unless you are painting the Sistine Chapel, there isn't room for more that one big idea in a painting. Communicating it clearly is the result of strong and simple composition.