And I'm no sailor. But! One of the most effective analogies I have come across that informs how we can think about success metrics is from the book, "Future Ready." This is an excellent book that is all about business forecasting. (It's also a great resource for any marketer that is also thinking about what success can look like.)
The authors make the point that when it comes to forecasting, some (or is it many?) organizations rely upon a traditional budgeting mindset. When they encounter a variance in regard to the original budget someone inevitably shouts to "get back on plan." But a budget - like a marketing plan - is a snapshot in time. It is probably a great record of our thoughts at that time, but what the variance is telling us is that as we move through the budget cycle, things may change.
So, when it comes to managing your marketing effort - think like a sailor. Your marketing plan is more like a map. Use your map to help keep you informed as to where you are and where you want to be. Then identify those success metrics that can serve as important measures that help you understand if you're on or off course. And like the sailor that encounters a change in wind speed, use your success metrics to help you understand what you need to do to change course so that you can arrive at your destination. Just don't worry so much if the journey doesn't happen as planned.