It doesn't have to be that way, of course, if you put in the required training over several months. This requires commitment and motivation, and a belief that the accumulation of all those training miles will help you to reach your goal-- whether this is time-oriented, or the equally valid desire to get round.
So what is meant exactly by motivation and commitment? Motivation is the desire to achieve something unusual that gives you the incentive to do things that other people are not prepared to do. Not many people are prepared to train week in, week out so that they can stand at the start of a marathon and say to themselves, "yes, I am ready and I couldn't have done anything more."
You can always do something more, but even the elite athletes feel that way. Every athlete has their challenges -- whether it's getting the children ready for school, or an Achilles injury. It's how we overcome these challenges that make us stronger, and mean we can stand on the start line with a quiet confidence.
If you're working full time you might have to get up every day at five a.m. to fit your training in, and this will take commitment, motivation and determination. It won't help if after a few weeks you revert to your former habits, and get up half an hour before you have to leave for work.
Whether it is getting up early, or beginning a fitness program your motivation and commitment needs to last over the long term to have an affect. In training for a marathon it will not help running for ten miles every two weeks, and doing no exercise in between. Your body will have forgotten what it has to do by the time you run again! It is better to run three or more times a week with one longer run on the weekend, and you will be in better shape.
These small, regular runs will add up to success over time, but they are far easier to do when you have a target to aim at. This can be a time you want to run, or the desire to complete the 26 mile race. Either way if you know what you want to achieve you will be more likely to do the things you need to do. Having a target concentrates the mind, and increases your motivation.