daf bit: Chullin 123

If one has become ritually impure through contact with an animal corpse (the g'mara specifically discusses tanning hides), he is supposed to immerse in a mikvah before doing certain acts. But what if there is no mikvah nearby? R. Abbahu said in the name of Reish Lakish: for kneading bread (for the levites?), for prayer, and for washing the hands before meals, if he can find a mikvah within four mils he must do so and otherwise he is exempt. R. Yose ben R. Hanina said: this applies only for going forward -- within the next four mils -- but he need not turn back even one mil. From this R. Aha ben Yaakov concluded that if it is a mil he need not turn back but if it is less than a mil he must. (123a)

(A mil seems to be approximately a kilometer.) From the explanations of R. Yose and R. Aha I infer that we are talking about a traveler, for whom backtracking would be an imposition.