What about, instead of doing something of that effort level (and energy system) that is broken up in the middle, something like 4×10' (no breaks) or 3×15' (no breaks) of progressively increasing effort level or pace (i.e. half-marathon to 10K to 5K)? This is a staple workout of the Japanese marathoners,
I heard/read about these type of workouts, but was not able to work them into my schedule (yet). Just to make sure I understand the concept correctly, depending on your target race you probably select the maximum pace of the last segment. So 5K pace is more for 10K/ half-marathon and 10K pace for a marathon?
That would likely depend on how limited you want your range of racing ability to be. There is no good reason that I could think of for someone who trains for marathons to not touch on 5K pace in training. The real idea is to increase the effort and thus increase pain and discomfort to mirror through training what is encountered over the course of a race.
Another type to consider comes from Vigil's notable 'critical zone' training (i.e. what Keflezighi, Kastor, and Hall do): LT runs. These are a bit more 'technical' in that they require sticking to a certain pace (starting out at between two and four miles) for as long as possible. The LT pace is defined as mile PR + 40″ (for high schoolers and everyone else; mile PR + 35″ for elites). To do this, one either would have to go to the track or have a marked course (perhaps a 1-5 mile loop with 1/4 mile marks) or one would have to get one of those pacer watches or (worse yet) one could perhaps do it on a treadmill, though the benefits would seem to be more limited there than for the other two options. The basic idea is to start out running the pace for a few miles and then gradually increase the distance, the workout portion of the run ends when one falls off pace. Kastor, Keflezighi, and Hall all started out doing this pace (mile PR + 35″) for 4 miles and, over the course of years, have progressed to the point where Kastor (5:01/mile) can now hold it for around 15K and Keflezighi (4:39/mile) and Hall (4:34/mile) can both hold it for over 15 miles. This should be progressive over the course of the training cycle as well as the course of the running career.
The other part of Vigil's 'critical zone' training is the AT run (i.e. long run), which is done at LT pace + 20″ (i.e. mile PR + 60″) — or LT pace + 17″ (mile PR + 52″) for elites. Again, a marked course, pacer watch, or (at worst) a treadmill would be a necessity for this.