Do the arithmetic once and it changes how you think about writing: five hundred words a day is a hundred and eighty thousand words a year, which is a novel with room to spare. The number is not the hard part. The hard part is still being there on day forty of a December cold, with no inspiration and a deadline breath at your neck.
So engineer the practice to survive bad days. Lower the floor until it is almost embarrassing — fifty words counts, one sentence counts, because the habit you are protecting is identity, not output. The person who writes badly for ten minutes is still a writer; the person who waits for a free afternoon is becoming someone who used to write.
Protect the same slot daily, keep the session short enough to leave you wanting more, and end mid-sentence so tomorrow starts with momentum already in place. Track the streak somewhere visible. Missing once is an accident; missing twice is the start of a new habit — the one where you do not write.