Before reading this I want you to do a simple test.
You have a current half marathon running PB of 1 hour and 50 minutes.
You do a 5 kilometer running test.
Based on a well proven formula that has been used to coach Olympic runners and world record holders your coach tell you that you can run a half marathon in 1 hour and 39 minutes (4:41 minutes per kilometre) and tell you to run at that pace for a half marathon event this weekend.
Your IMMEDIATE response ?
About 10 percent of people, in my experience, answer with just one word - OK. Of the remainder about one third say "I'll try" and the remainder answer "I'm not sure (or I don't think) I can do that"
And there in lies the delimma
For many people, in order to do something, they have to know or believe they can do it. The problem is that, if that is the case, they will never consciously improve their PB. They 'know' they can run 21.1 kilometers at 5:13 pace so they can do it. They dont know, or havent proven, they can run that distance at 4:41min/km and therefore dont believe they can. This is especially true of milestone numbers like sub 1:30 in a Half Marathon or Sub 3, Sub 4 hours in a marathon. Studies of result in runs show clumping of results around these milestone numbers
If you have to believe via proof that you can achieve something in order to do it then, by default, you will never set a PB.
The difference with the 10 percent of people who say OK - and in my experience then typically achieve what they are asked to do is not that they 'believe' they can do it. After all they technically have no reason to but they a) trust in the information or data given to them based on the results of studies or others and/or b) do not doubt they can do it. There is a big difference between self belief and lack of self doubt and typically the later is more important in achieving Personal Bests.
Quite often runners or endurance athletes are very analytic which is both a good and bad thing. Bad in that over analyse things or are governed by their own imperical evidence. Evidence is, by definition, what HAS happened and therefore what CAN happened is definined by this evidence. It is interesting, without getting into a religious discussion, how many people believe in things like God or life on other planets with NO evidence to actually support this yet are massively sceptical when it comes to over things.
If you are not one of the 10 percent or one third that say "I'll try" then start to look at whether your need to 'believe' is actually limiting your performance. Then ask what do you need, outside of the performance itself, to remove any doubts. It could be talking to one of the 10 percent who have improved their PB's based on the same system that is being used to set your goals. It could be a case of moving to being one of the 30 perfect and saying "You know what - I'll give it a try" - maybe do this in training rather than an 'A' race if you're not 100% committed.
But the simple, limiting fact is that if you need to do something in order to believe it's possible then you will never set a PB.
Hence the PB delimna.