На большом пути и малая ноша тяжела

Even a slight effort one keeps making for a long time makes him weary. See Усталому коню и сбруя тяжела (У)

Var.: На сто вёрст и иголка тяжела

Cf: On a long journey even a straw is heavy (wieghs) (Br.). Too long burden makes weary bones (Br.)