Speed First! Yes you must work on speed first and foremost. It must be part of every training cycle. I find it quite amusing when I hear a runner say that I have been working on base work, but I have not started speed work yet. The problem with that approach is that they are not training to run fast, they are training to run far, and they hope that the fast will come. The inevitable result is undue soreness and greater risk of injury because of the abrupt change in the training program when they do start to run fast. The key is to never get too far away from running fast. It should be part of the first training cycle of the year and be a part of each subsequent training cycle. Speed development work can be as simple as sprint drills, light acceleration drills, or simply finishing each run with 8-10 x 100 meter fast strides.
It may be a coaching cliché, but the winner of the race is the person who slows down the least. Therefore I think it is helpful to think of running at a percentage of the individual maximum velocity. The goal in training is to continually strive to run longer at a higher percentage of peak velocity. Rather than focusing on pace, it is better to focus on distribution of effort. Races at any level are seldom run at the physiological ideal of even pace. The goal should be to distribute the effort as efficiently as possible over the entire race distance.
It is interesting to note that the highly successful Moroccan school of distance running clearly acknowledges the importance of speed and power in distance running performance by their talent identification test. They test a short sprint from a standing start, a middle distance race and the standing long jump.