The two hour mark may be the goal in sight for elite marathon runners such as current record holder Paul Tergat, but for many of us 'mortals' a sub three hour marathon is an achievement not to be sneezed at. Aiming for a sub three mark is certainly within the range of any fast runner below 35 who's done a few marathons already, but unless you are one of those people endowed with exceptional natural fitness, it definitely requires a step up in terms of the amount of training involved. If, so far, your training for running marathons has been based on a 'hobby' approach which had to fit around anything else that was going on in your life, then you might need to change priorities a little.
In general, you are looking at going out there for a run an absolute minimum of five days a week in the ten weeks leading up to the marathon. There is a school of thought out there that says six days on peak distance weeks, but certainly five is a minimum. The good news is that not all of this is hardcore push-yourself- to-the-max running, in fact relatively little of it can be with the right schedule. A good schedule will contain some intervals and temp runs almost every week but these will only just slightly push you outside your comfort zone, not destroy you completely.
In fact the only time you should be pushed to the absolute max during your training schedule is during the lead-in races. I would recommend at least three of these: a 10k seven weeks before the race (target time 39:30 mins), another 10k five weeks before (target time 38:30 mins) and a half-marathon three weeks before (target time 1:24). I know some friends of mine have suggested an even quicker half-marathon time, such as 1:22, but I'm going by the old "marathon time equals twice your half marathon time plus eleven minutes" rule. Try and pick out races that correspond to these times (plus or minus a few days is ok), and go to them even if they are a distance away. You can bring some newness into your training by combining it with a trip away, and running races always makes you feel more 'professional' about your approach.
If you can't find a race corresponding to your planned distance at the time, get out and try to do the distance in the target time anyway. Try and mentally create the conditions for a race - perhaps do a course you previously ran a race on (especially if you ran a good race that day!), do a trip around in the car and place water every few kms. Mentally shut out the feeling that "this is not a race and you won't do as good a time" - really feel that you are racing against the stopwatch.
Of course the other big aspect of training is the long distance runs. Instead of building up to having your big three-hour run three weeks before the event, try to have it so that you build up to a two-and-a-half hour run early on, perhaps even eight weeks beforehand, and then you can your first three hour run six weeks before hand. If you have run marathons already, your body should be able to take the upgrade (however if your known susceptible to overtraining injuries, then by all means grade your long run schedule over a longer period). In this way you can have up to three three-hour runs in the course of your schedule. One good idea to have with your runs is to start off slow, and then gradually speed up so your las twenty minutes is at marathon pace. This will help your body reflexes at mile twenty in the marathon - instead of slowing down, some part of you will want to speed up!
As for the race itself, if you've done the training, you only need to watch out for one thing - don't go out too fast. You can start out at 7:30 per mile for the first three or four kilometers and then slowly increase to marathon pace. If things get tough, remind yourself of all the training you've done. You did everything the schedule required, did all the races, you're more than ready. And most of all: enjoy the moment when you crossed the finish line, you worked hard for it!