That is so pessimistic and suggests poor estimation and general budgetting capabilities.
Take the speed that you find comfortable/reasonable for a 3 hour ride unladen (refer to your rides done in the last few months), and then subtract 1 mph for every 5kg carried (ie baggage and carriers) and 0.25mph for every hour (rolling) over 6. This formula is, of course, designed to deliver an estimate for planning purposes and will vary from person to person. Terrain (ie climbing), difficulty of navigation and use of narrow roads and likely (high) temperatures are also worth taking into account.