In addition to the running that you are going to have to do in order
to complete your first marathon, it might be a good idea to include some other types of
exercise, either to achieve specific goals (eg injury reduction) or to vary the training that
you are doing.
The time on your feet running is far and away the most important part of training for
any beginner attempting the marathon, but there are other forms of exercise that might help as well.
Cross Training
If you are susceptible to injury, or are not used to lots of running, it might be worth
doing one of your aerobic (fitness) sessions a week in another way. This might be swimming
or cycling or on a rowing machine.
This has the disadvantage that you are training training your muscles specifically to run
but, the advantage of this is that they are not placed under undue stress, which could be
an advantage as they recover between running sessions.
You will still be training your heart and lungs as you do this sessions, so they can
prove to be very useful.