Planning it to be like a novel with an ending can work. However, I find if it's longer than a small novel, 90,000 words, I tend to lose steam.
The two things I try to avoid if I'm writing something lengthy is Over-Worldbuilding and Over-Planning.
Too much worldbuilding because it's easy to get trapped in endless minutiae and spend hours and days thinking of stuff that isn't useful to my actual writing and end up with no creative fuel left over for the actual story.
And over-planning because planning out every little granular bit of the story can lead to a feeling of writing by rote which for me isn't as enjoyable as writing and being creative at the same time, having to think things up on the fly.
These are both kinda counter-intuitive to writing but if you have problems with staying motivated and excited to write I'd say they are helpful, they have been for me anyway.
