Hey Sean,
The idea of that test is to test how well you are improving at increments of 10 bpm, with the main focus on that final tier that is just above your marathon HR, which is where your lactate threshold usually resides if you are well-trained. If that tier is improving, and you are feeling comfortable, then that is a good sign. If your LT is in bad shape, then that level will feel very uncomfortable. It might be wise to run them by time instead of by 2400m. Take a look at Joe's times for each level, and try to mirror the time instead of the length.
Some believe that the whole building game that Hadd lays out is not necessary and that a 20-40 minute weekly lactate threshold tempo run (starting with 20 min) at 88-90% of MHR, or somewhere around 15k to half marathon pace, is all you need, in addition to everyday aerobic training. Some will alternate the LT run with cruise intervals every other week. You can measure your LT progress in those weekly runs.
--Jimmy