A competitive runner's career has four phases and how you train is not terribly important in all of them.
Early in one's running, no matter what you do, you don't seem to improve very much. It takes a while for your body to respond, to say... "oh I get it, you're going to keep doing this." A lot of people quit because of this. Others live in this phase - the biggest change in running as a sport since 1980 has been the development of the true recreational, non-competitive runner who still enters races.
In that first phase, you might get a surprisingly good race result and then start thinking you might have a future in racing. You start racing more often, hoping to repeat that result. You start thinking about what you could do to run faster. In this phase, which on the average lasts about 4 years, you improve rapidly, regardless of how you train; every workout is new enough that it brings results. This is where runners always make the mistake of post hoc ergo prompter hoc, thinking that the specific things they've been doing in training have brought specific results. Runners will tell you that they've found THE training plan to run faster, because they followed it and they got faster; if it stops working, they switch to other plans until they improve again and then they tell you they've found the REAL way to train. They think their coach is a genius because they've improved under their tutelage, when the coach usually is just making things up day-by-day.
If you're in this phase, enjoy it! You'll spend the rest of your running career trying to recapture the days when you seemed to improve effortlessly.
Once improvement starts slowing, real training begins. You get less and less with increasing workloads. You have to train specifically, you have to plan carefully for a few particular races. This is where training can start seeming like a chore and you have to decide how much effort you want to put in to cut off 1 per cent from your finishing times. This phase, where you're in peak shape, lasts maybe 10 years, if you're lucky.
Eventually, nothing gets you to improve any more and you struggle to hang on to what gains you've made. Training harder just becomes overtraining and you lose fitness. Train less and you lose fitness. Train differently and nothing changes. The decline is slow at times and age-related - nerves fire more slowly, recovery takes longer, etc. - and sometimes is rapid and injury-related. Your fastest races are behind you. A lot of competitive runners, perhaps most, retire at this point.
Those who continue, however, may find that their best races aren't necessarily their fastest. It takes a change in mindset that's difficult after a long racing career. I hope that's where I am.
2 days ago