聚沙成塔，集腋成裘

For most general‑purpose writing, 千里之行，始于足下 is the best fit.

 

- Original English: Inch by inch is a cinch; yard by yard is hard.

- Chinese proverb：千里之行，始于足下

- English gloss: A journey of a thousand miles begins with a single step.

 

It conveys the same idea: big goals feel daunting, but taking small, steady steps makes things achievable.

 

Short alternative if you want to stress accumulation of tiny efforts: 聚沙成塔，集腋成裘 — great results come from many small bits.