Sounds like a normal job in the private sector, and the stress issues of having to get to the next parcel in x number of seconds sound quite trivial.
Nothing further from the truth.
If you have a contract with notice etc. trying to compare it with agency, where they can and do fire on the spot!
As for x seconds, they will have calculated how long it takes, want a drink tough. Not sure what they do about toilet breaks, but have known places time people on that one. Too many or too long and your fired.
TBH the mileage walked isn't really an issue, you do get used to it. Place i worked the job was walking back and forth 5-8m on the end of a steel mill for 8 hours. Concrete floor steel top cap boots what fun!