The problem is, if you want to do something you've never done before, then the steps to that, the milestones, will be unfamiliar. Analogies are useful for explaining something unfamiliar in familiar terms. The most important thing you must do to arrive at any destination is start the journey. In order to go somewhere or do something that you have not done before, you will need to step out of your comfort zone. Stress and fear are normal reactions to the unknown.
Many people are afraid of the unknown.
By trying to do something that you have never done before, you are heading into the unknown. Again, it has to do with the way the mind seeks out connections - associations that it can categorize and fit into its system of beliefs. It is uncomfortable going past your zone of knowledge because your brain needs to work harder and has less information that fits easily within its frame of knowledge. When the brain encounters the unfamiliar our brains may interpret new feelings as anxiety or depression or fear. You will not know at the time, which of these feelings are actually conducive to success.