Hard – easy – hard is one way of describing the plan sequence. A better way would be to look at the physiological focus. The first plan is neuromuscular power, VO2max and supra threshold, the second plan is primarily aerobic (which “could” be called easier as the power level goals are lower) and the third plan is race specific abilities.
To add in 6 weeks to your sequence the best place to do this would be to add more aerobic base training by repeating the last 6 weeks of the base training plan.
Lynda