Of course hoping for the best is all well and good, but it might not see you reach your potential in terms of finish time. Make no mistake, 26.2 miles is a challenge for even the most accomplished runner, but planning how you'll handle the race itself can go a long way to helping you get the best out of yourself, reducing the element of chance and allowing you to actually really enjoy the event.
It's advisable to have an idea of your potential finish time but not to obsess over it. It is fairly common to take quite a while longer to complete the race than your anticipated finish time; often you base your estimate on your longest run to date which may have only been 20 miles. Don't be fooled into thinking you will run the last 6 miles at the same pace. They will generally be much slower as your body is tiring and muscles begin to ache.
Anyway, you won't get the most out of the race if you worry and fret about running it in a certain time. Unless you are a pro-runner and attempting to beat your current personal-best, don't stress too much about timing. Getting round the course (especially if it is your first marathon) is a huge achievement in itself.
If you are determined to set a target, make it a realistic one, and err on the side of being conservative. Give yourself the best chance by not setting an impossible target time and unsuccessfully chasing it for the whole run.
It is essential to have in mind a plan for the race before you begin. It is doubly important that you stick to it; too many runners have let adrenaline take over early on in the race and decided to push themselves on a whim, only to have to walk the majority of the second half of the race. One of the biggest potential traps is to be heavily influenced by those running near by (both at the start and during the run).
At the Start of the Race
You must be sensible and go off very slowly at the start of the race, You will often have no choice as the congestion will be such that you can't move very fast for a while. Don't let vanity take over and try to keep up with others around you; if they whizz off at super-speed they will usually regret it further down the line. Go no faster than the pace you have trained at. Be smart and conserve energy for when you'll most need it; the last few miles will be challenging enough without having burned yourself out.
During the long runs in your training, you will have settled into a maintainable pace. If you deviate from this wildly on race day, you are bound to struggle. Keep constant. This can be easier said than done when there are thousands of people cheering; be prepared for this and don't be seduced by the rush. Of course you want to enjoy the atmosphere of the marathon, but don't expend too much energy engaging in high fives with the crowd and whooping with delight at every opportunity. Remain in control. It's all too easy for the early stages of the race to become a blur of racers darting by and for you to get carried away at a speed that is not optimum for your success.
After around six miles the field will stabilise a little and there will be a bit less movement in terms of people over-taking. The novelty of the event will still be there, but it should have worn off a little; the distractions will be far easier to handle, allowing you to focus and absorb the atmosphere at the same time.
There's no real need for a running watch if you are a first-time marathon runner, because at each mile marker there will normally be official race clocks. However, it's important not to pay too much attention to this time as it will denote when the gun went off not when you crossed the start line (which will be recorded on the chip attached to your running shoe). It is only really useful for letting you know how long each mile is taking; again observe your distance times but don't let them rule your race. Try to be as consistent as you can and keep a regular, steady pace.
As you go into the second half of the trace you may start playing mind tricks on yourself. You must now employ the technique of mind over matter. If you have trained properly and done your fair share of long distance running, you may be familiar with the constant conflict in your head. One voice is telling you it can't be done and that you must stop, whilst the other is urging you on. The most challenging part of the course is normally around miles 16, 17 and 18. Do not let the negative voice overpower you at this time. Avoid walking for long periods of time and dig deep when your morale starts to fail.
Running out of energy at this point is common; your glycogen stores are seriously depleted. You can counteract this in advance by eating plenty of carbohydrates in the days before and training properly.
The end is in site
If you are competing in one of the massive events like the London or New York marathon, the crowds will be a huge motivating force in the last few miles. Most beginner training plans don't push you past the 20/21 mile mark, so you will be on new territory by mile 22. Use the final scraps of adrenalin and the roar of the crowd to inspire you at this difficult juncture. Many runners find that they experience an almost renewed energy at this point which carries them through to the enormous thrill of the finish line.
Other articles of that may be of interest:
What to Eat and Drink on the Day of a Marathon
A look at the best nutrition and hydration for runners on race day
Have You Got What it Takes to Run a Marathon?
A run through of the most basic training commitments needed to run 26.2 miles