Keep in mind that my guys go directly from the the tempo rep to either push ups or some sort of abdominal work. Then 45-60 sec rest and another run.
Most important, is that there are divisions of improved cardiac and aerobic function relative to anaerobic threshold.
One is in error to assume that the workload must be (insert your favorite adjective) difficult in order to improve aerobic work capacity.
The work capacity must be defined.
+108% anaerobic development
102-108% anaerobic maintenance
94-102% anaerobic threshold = aerobic development
85-102% aerobic maintenance
68-84% cardiac output
60-70% cardiac recuperation
Remember that tempo, from a bioenergetic standpoint, is performed in order to improve aerobic qualities.