Training for a marathon, especially your first is not like your regular running training! Most regular runners can get up to half marathon simply through regular run training, however the step up to marathon requires something different.
The first thing to consider is your shoes. You will be putting considerably more load on your feet and legs than you may be used to, so want shoes with a good amount of cushioning. If your current shoes are well cushioned they may be fine, but if more focussed on responsiveness and weight for shorter runs it is worth considering a more plush shoe. Running longer distances on too firm a shoe runs a risk of injury. Also keep an eye on the condition / mileage of your shoes as you will be increasing mileage considerably and don’t want to out run their useful life just before your marathon and end up running it in shoes you haven’t trained in! Manufacturers typically recommend 300 – 500 miles per pair.
Be realistic about the amount of time you can dedicate to training, but be aware that you probably need to be looking at five or six sessions per week to build sufficient endurance in your legs. This will also impact on the training plan you select.
The most important thing you need is a plan! There are many plans available, the key thing is to find one that works for you and stick to it. Make sure your plan includes a variety of running workouts including long runs, easy runs, and interval work. Also remember to do some strength training to help build muscle in your legs and reduce the risk of injury.
Key features are a mixture of general fitness training (cross training in lower plans, speed work in more advanced), endurance building long easy runs and pace runs to get you used to running at your target marathon pace. By for example doing a mid-distance pace run on a Saturday, followed by your longest run on a Sunday you also get used to running on tired legs.
Perhaps the most surprising thing about marathon training, is that you will typically run to a maximum of 20 miles in training. Most plans will typically include from one (at novice end) to three at advanced end runs of this length. It may be tempting to try full marathon distance as part of your training, but the problem with this is that when running this full distance you are drawing on your deepest reserves so it will take too long to recover. Better therefore to save this for race day, when the atmosphere and support will carry you through those extra 6 miles.
My final piece of advice is to observe the taper. This is where you substantially reduce the distances over the final two weeks before your marathon. It feels odd and can be very tempting to go out for a longer run, but this period is important to allow your body to build up the reserves to get you through those final few miles. Provided you have trained to the plan, you will not lose fitness over this two weeks and will be raring to go come race day.
Enjoy your marathon!