But the problem with the weekly training unit is one hiccup in you week and the target is gone. Life can easily put your target off track - the once in 3 months event, the parents evening, the family event - all can knock your weekly target off track. And because its just a week you are working with there isn't the time to reschedule - the week is over almost immediately.
Using a month as your basic training unit is much better, I have found. Lets take the idea of 80km per week target - that's 320km per 4 weeks. If we're going to have a rest week lets do just 40km during that week, so 280km left to distribute across the remaining 3 weeks.
OK 280 divided by 3 = 94km per week, so a 50km and a 20km at the weekend plus a 25km mid-week would do it. BUT one weekend includes a sportive you want to do - 80km, and another weekend is going to be a family visit with no chance to do any riding (its your sisters wedding so don't even try). One of the weeks includes a Bank Holiday Monday. So your 3 weeks includes a mixture of opportunities and limitations and you can plan accordingly to get your month target reached.
Besides helping you cope with real life this way of planning your training can also lead to inventive solutions and grabbing opportunities: days when you finish work early or make the effort to get up at dawn to knock off the final 40km. It can also mean that you have the time to push yourself - you've got 140km to do in the last 5 days of the month - so, despite being pretty tired already, you push yourself to get that final 140 done.