Before reading this, you should know that I am not a doctor or medical professional. If you experience excessive discomfort or pain while writing, seek professional help.
Writing for long periods of time (over an hour) comfortably and without injury can be very beneficial if you take notes, journal, write letters, or write anything more than a shopping list or a quick note. Although it's true that every person writes differently, there are certain rules that can greatly improve every person's technique for writing for long periods of time (marathon writing).
Posture and Position
The first rule of marathon writing is to always maintain a comfortable posture and position. The more comfortable you are, the longer you will be able to stay in the same position. Try experimenting with position allows you to write for the longest amount of time, most comfortably. The most commonly recommended position is a straight back with feet resting flat on the floor and arms resting on the desk. The desk or writing surface should be high enough to prevent neck strain. Always remember to bring the paper to you, not you to the paper.
The second rule of comfortable marathon writing is to have a light pen grip. One of the huge advantages of fountain pens is that they allow you to grip the pen very lightly and use very little pressure on the paper for the pen to write. Ballpoint pen users often have trouble using a light grip when they switch over because of the pressure a ballpoint requires to function. When using a tight grip, the small muscles of your hand and forearm can easily strain and cramp up, greatly limiting the length of time you will be able to write. If you are having trouble keeping a light grip, practice by forming letters lightly and slowly and gradually get faster until you work up to your normal writing speed. Another tip that will reduce the possibility of overuse injuries is to move your wrist as little as possible. Try to use your fingers, not your wrist.
Pick the Right Pen
The third rule is to select the right pen. As you might have gathered from the last rule, it's fountain pens are the most ideal. The pen you use should be comfortable to hold and shouldn't slip when your hand is wet as moisture can easily build up as you write. It also should be heavy enough to write under it's own weight but not too heavy that it might cause fatigue. I prefer my pens to have a wide grip section as it helps me to keep a loose hand and control the pen more precisely.
Take Frequent Breaks
The fourth rule is to take frequent breaks. I usually take a break every 20 minutes to stretch and loosen my hand unless I'm going to forget something if I don't write it down. If taking breaks is difficult, consider using a pen with a smaller ink capacity. You can use the time it takes to refill it as a break. My breaks usually only last for about a minute.
The last of these rules is to have an eye-friendly environment. Pick a well lit, but not overly bright area for your writing. Also, pick a muted color (such as a darker blue or black) ink and paper to avoid tiring out your eyes. It should be easy to see even the smallest of words on the paper with out squinting or straining.
Remember, if you maintain a comfortable writing position, use a light grip, use a comfortable pen, take breaks, and keep an eye friendly writing space, you should be able to marathon write without causing any injury or discomfort. If you do experience anything unusual, consult a medical professional. I hope this has been helpful in allowing you to marathon write.