Phillytom,
I see the faster ones as more of a peaking workout to take you all the way there. Doing a lot of threshold work will take longer but I believe you will reach a higher peak and it will last longer. If you rush into a lot of hard stuff, you will be racing faster sooner, but progress is likely to stall sooner.
I'm an old man now but have been progressing for the past 3 years. My last two 5k's have been breakthroughs, and they came during HM training--stuff like 1000's at 10k pace, 6 x 1600 at HM, and long tempos at MP. I guess that's where a lot of my bias comes from, but I've seen this sort of thing happen with other runners too and have a good coach advocates a more gradual, steady progression rather than the quick fix approach.
From Steve Scott:
"During my career, the most important aspect of my training regime was strength. You can spend the rest of your life working on speed and make little progress, but if you spend the rest of your running career working on strength, you?ll always continue to improve. Improvements are also achieved by increasing your endurance. What do I mean by strength or endurance? Mileage, Mileage, Mileage! The more miles you can run, the stronger you will become."