The gemara says that for tefilah and netilas yadayim one must travel 4 mil forward, and just under one mil backward. Rashi explains that we are talking about someone who is on the road travelling and he wants to stop for the day, but knows that he will have a shul 4 mil ahead to daven (Tosafos assumes that this means a minyan, not a physical structure), he must continue to go to the shul. Similarly, if one was hungry and wanted to eat bread but didn't have water to wash his hands, he would be required to wait until he travels 4 mil before he eats.
The Biur Halacha (163) assumes that the shiur of 4 mil is really a time requirement. Since it would take 72 minutes for the average person to travel 4 mil, the gemara is demanding that one wait 72 minutes. A possible scenario would be if one is on an airplane and wants to eat bread but can't leave his seat at the moment, since it is very likely that within 72 minutes he will be able to wash, he is obligated to wait. It isn't clear to me why the Biur Halacha assumes that this would be dependent on time. Rashi uses language of "tircha" implying that one must push themselves to continue to travel, implying that the amount of 4 mil was for a circumstance where he was on foot and walking 4 mil is a significant tircha. Perhaps if he were travelling with minimal tircha he would need to wait longer than 72 minutes.
The M.B. also comments that if one isn't certain that by waiting or continuing to travel they will find the shul or water, they aren't required to wait at all. This is certainly the implication of Rashi who writes in definitive terms that there is water and a shul in front of him.
Another point that the Biur Halacha makes in the name of the Chayei Adam is that if one isn't very hungry he should wait even more than 4 mil. However, Rashi gives an example of one who wants to stop while it is still day and the halacha demands that since it is still day time and easy to travel, he must continue. It would seem that a similar case by eating would be when one wants to eat now but can relatively easily delay. Perhaps if one is very hungry so that delaying is very difficult (on the level of travelling at night), they wouldn't even have to wait 4 mil.
Another point that the Biur Halacha makes in the name of the Chayei Adam is that if one isn't very hungry he should wait even more than 4 mil. However, Rashi gives an example of one who wants to stop while it is still day and the halacha demands that since it is still day time and easy to travel, he must continue. It would seem that a similar case by eating would be when one wants to eat now but can relatively easily delay. Perhaps if one is very hungry so that delaying is very difficult (on the level of travelling at night), they wouldn't even have to wait 4 mil.