You can train intensely or frequently, but not both. The harder you train, the more it takes out of you.
Assuming that you're training for the sake of performance, the correct philosophy is to train to race, not train to train (i.e. to condition yourself to absorb X mph just for the sake of the mileage). Training is means to an end, not an end in itself.
The most efficient way to train is to conduct maximum-intensity training sessions that closely replicate the exact demands of competition. You run these sessions as hard as possible, rest thoroughly, and repeat. The exact opposite of how most runners train today, where the goal is to stack as much low-grade activity on top of low-grade activity as possible.
In summary, if you train like people on this board say you should, you are bound to end up overtrained and injured.