Fast and short vs slow and long

I've come to the conclusion that the only way to better my half and full marathon times is to drop my training pace to a faster level than my goal pace. So a 6:30 for 7 miles instead of a 7:10 for 14. I'm not sure whether I'll be able to do this, or whether I'm right, but I feel like I have plateaued at the 7 and change mark and I don't think I'll be able to make a lot of 7 minute pace 16 mile runs over the next few months. I just don't have the time. So my new short term plan is shorter and faster. This may make the running too unpleasant - I might not be in shape for 6:30s. Guess I'll just have to try it and see.