Alright, I might be wrong because I'm new to this whole strategus thing, but my understanding is that currently, you can only transfer troops and equipment to people are who right on top of you on the map. I gather that this was changed from the previous system because people were transporting troops and equipment instantaneously across the map using a relay system with multiple characters.
My suggestion is rather than reducing the distance across which you can transfer, simply make it so that each transfer is not instantaneous but takes a set amount of time to process, say an hour (or whatever). In this way, you could still relay supplies faster than one person walking the whole way themselves, but it would not nearly be instantaneous, especially across greater distances.
Possible implications of this suggestion:
1. If you can find a group of people you trust, you can transfer supplies faster by co-operating, than you could alone.
2. Supply lines become a strategic consideration, and maintaining a constant supply line between fiefs would be important.
3. Supply lines can be a target of an attack to disrupt them (what actually happens when to an army when it is defeated?), providing more opportunities for open field battles as opposed to sieges, as well as a role for smaller armies.
4. Longer supply lines would be more vulnerable than smaller ones, sparser ones more vulnerable than more redundant ones.
5. Supply lines or the protection of supply lines could be contracted out, providing a potential role for smaller, fiefless factions.
6. Anyone involved in the supply line has the opportunity to defect, whether by abandoning the supply lines, transferring the supplies to a third party instead, or what have you, creating more possibilities for espionage and under the table dealing.