With watercolour it is generally true that what you put on the paper ...stays on the paper. It is very difficult to cover up mistakes or change tack half way through a painting. Now you can do this with oil or acrylics, so you can get straight on in and worry about the details as you go along. Not so with watercolours!
It is a must to think about how you are going to approach your painting and this 'thinking' stage can take a long time especially if the subject is a bit complicated. I very rarely start to paint till I am 80% sure how I will finish it - this reduces the risk of making mistakes so I can concentrate on the painting process itself. A good way to start is by making small pencil sketches of your composition so that you get that right before moving on to a large sheet of paper - make your errors on scrap paper rather than your painting paper. It is always good too to walk away every so often, to stand back and take a look from a distance rather than close up. Never try to finish a painting if you are tired - far better to leave it and finish it tomorrow.
So have fun with your painting no matter how long (or not!) it takes you.