We made an itinerary when we first started our trips. Probably did it the first 3-4 trips at least. We did pretty much stick to it. Wouldn't have been a big deal if we hadn't been able to but conditions typically allowed for it. I think it is helpful to plan it out, especially for your first trip - go through the guide books, figure out the places you want to stop. Just don't feel you have to stick to it. Prioritize the places you really want to get to and the places that you'd like to but might not be as important.
Even when we've pretty much stuck to an itinerary, we have made adjustments here and there. Being flexible is key.
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