You can start the outdoor activity from any point on the route as long as you are on the itinerary. However, make sure to follow the intended direction, in other words, from the starting point towards the finishing point (indicated by a black flag).
If you are too far away from the route, your position cannot be aligned with the finish. In this case, the app will show your position as off-course (orange arrow). You will need to join the route so you can make use of the navigation. Ensure, therefore, that the arrow pinpointing your position is actually on the itinerary. This way, you can make the most of your outdoor route!