)In my old office if a full timer left/retired then the duty they were on would be advertised to the staff and the duty filled. The duty holder that took that duty had their duty advertised and filled and so on so forth until an advertised duty was taken up by a part timer (or if no-one applied a new starter/reserve). This method can (not in all cases, bt usually) gave the senior staff a (well deserved imho) chance to get a nicer/less difficult duty.
I've since transferred (stayed FT) - I've been in my current office for 6+ years now and I've not seen a walk advertised at all
. In fact the only two duties advertised were DOM support and a leadership role looking after the vans!! I got trained on a walk when I started and have been there ever since! (its a proper busy walk that no-one wants!!) There has been at least 5 staff who've left/retired but not one of the duties have been advertised, instead they've been given to new starts (part time with staff taken off IPS to prep them) or to staff who can't cope on busier walks!