It is good to know whether you are ready or not for, say, a sub three hour marathon. If the table and past experience do not project to sub three, wait a little longer before running the marathon.
I use the marathon as an example because most people can't just keep racing marathons. Each marathon requires that you take much longer to recover than is needed for shorter events.
"But I know people who race many marathons and they keep running good times" I hear you say. (I have very good hearing). But I contend that such people might run half an hour faster if they stopped racing marathons so much and did only one or two of them in a year. I say that because I have seen it.
Don't race marathons to see how your marathon training is going. Race 5ks or 10ks and use the time predictor table to see what you might have run. Then when you are ready, pick a fast marathon course and go for it.
Marathon training will improve 3k, 5k, and 10k times significantly. And every time you improve over the shorter distances, you can be certain that your potential marathon time is improving proportionately. Note though, you must actually be doing the long distances required for marathon training for this to be so.
If I ever get under 38 minutes for 10k again, I will consider returning to marathon racing. Maybe. I think I enjoy 3k, 5k, 10k, 21k more than I enjoy marathons. A marathon is endured rather than enjoyed. So, maybe not, even then.
