I've seen several runners PR in the 5k or 10k when they give up 5k/10k specific training to focus on HM/M training. The thing that's interesting to me is that their mileage doesn't seem to change much, just the workouts. Most of these cases have been runners on moderate amounts of mileage, say 40-60mpw and some of them hardly bump their mileage at all and still see themselves PR'ing once they make the change. They cut out speed work (maintain strides though, I think) and rarely run v02 reps.
The workouts are weekly long runs, a long 'tempo' (Daniels M pace or Tinman tempo type stuff), and a normal tempo/cruise interval workout or 10k paced reps.
This is interesting to me. Curious to hear from any others that have seen these sorts of results. My only thought is that they needed more aerobic workouts than they previously were getting, or the lack of hard reps/v02 max work allowed them to avoid injury and stay consistent.