Probably the easiest way to simulate this would be to add a modifier to the recall time based on the current distance between the character being recalled and your ruler. I specify ruler instead of capital in the case where your ruler and the general being recalled may be leading armies together in the field far from home, in which case it wouldn't exactly make sense for his lordship to travel to your camp in Jerusalem by way of London first. I wouldn't say no to current location having a greater impact on gameplay, however. Especially in the case of a siege: if I win that siege, I want everybody there as my prisoner. I want lives at stake, and the option to try and send my family to safety if a siege seems near.