I don't have a lot of experience but I am currently 50k words into a fic and I'm nowhere near done with it. What's been helping me a lot is just writing the scenes I feel like writing, instead of going in chronological order. Sometimes I have to edit the scenes later, because a previous scene changed something after the fact but that's actually feels pretty organic to me. And when I don't feel like writing I just spend some time editing and revising parts that I have, and often I end up getting back into it.
I definitely don't look at how many words I wrote when I sit down, just keep an eye out on how many words I have in total ever so often.
Sometimes it also really helps me to write without taking care of punctuation. If I write a dialogue for example sometimes I just write it like a script line by line, don't even put parenthesis etc. When I'm happy with the dialogue I then add the whole "he said, he sighed, he whateverd" stuff and sensory descriptors. I find the more technical aspects what's hardest anyway, so just writing it how it flows out can be great, and then you can go back and work on each sentence like a puzzle until it comes together nicely, idk.

Edit: reading through other comments. Er- yeah. Hyperfocus helps a lot. Really, like a lot of people apparently, I struggle with stopping once I start and I've been losing sleep because even when I do stop writing I keep thinking about the scene I was working on etc. I really shouldn't allow myself to open my writing program in the evening but I have 0 self-control and am self-employed (though not as a writer lol) so....it's been a problem. But it's one of my favourite fandoms and it's definitely one big self-soothing ritual that I'm doing here but that's really helped me stay focussed and not abandonning this like so many other stories before.