I've been writing for a long time. It started with role-playing with friends over instant messenger, morphed into fan-fiction, mostly Harry Potter fan-fiction (I am not embarrassed to say; my fan-fiction was often highly commended) and resulted in perhaps a dozen short stories, a poem or two, and half a dozen novels in progress.
Of these, I have tried to spread the writing around- a single science fiction novel, a thriller, a fantasy novel, a drama, and one that can best be defined as essentially vampire teen angst-lit.
The novel that is the furthest along is the science fiction novel. I laid down the idea about a year ago, but I only actually started writing in earnest about eight months ago and took a two month hiatus during December and January; today, I have approximately 45,000 words or so laid down, and the stringent story-planning and outlining has been set down for, I imagine, close to 40,000 more, and, when on my stride, I can write between 2000 and 3000 words an hour, although I rarely sustain this for more than two or three hours at a sitting, as the effort is exhausting and it tends to come apart at the seams- the tone fluctuates and the consistency fades.
One of the most difficult problems I run into while writing is consistency; this is especially true in a science-fiction context. I spend an inordinate amount of time making sure that sizes, ranges, weights, masses, and various other figures given do not conflict with each other and fall inside the realm of possibility or even probability. I similarly attempt to ensure a consistency of context- if a piece of technology does A once, it should continue to do A, and nothing but A, unless a good reason exists for this ability not being shown or referred to previously- especially if it would have been helpful before.
However, this can be crippling at times. For example, the Pride of Albion, a vessel in the novel, is approximately 1100 meters long by 360 meters across the beam and 360 meters in 'height'. What is an acceptable mass for this vessel? First of all, one would have to determine the volume- no small task when the object doesn't exist in the real world or as an accurate 3-D model. Then, one has to determine the density of components that don't exist. It is in some cases possible to base a density off of real-world ships, but various in-universe explanations (like wide-empty spaces) must also be taken into account. The amount of work involved in ensuring that the mass of the vessel, a figure that may only be used once or twice in the book is consistent not only with itself but with the wider universe as portrayed is perhaps an hour, or two- or even more, depending on whether or not one accepts a very rough approximation for volume or attempts to model the figure precisely.
The second leading problem is tone. I often find that it is a struggle to ensure the tone is consistent throughout a work (unless I write it in one sitting, which is possible for shorter works but not for entire novels). Rather, my writing will have a specific tone on one occasion and a slightly different tone on another, depending on my mood; and more importantly and disconcertingly, if I am attempting to force myself to write (generally by giving myself a quota of 1500 words per hour) the tone often tends toward the flat and uninspired, with lots of description but no flair. Attempting to ensure that I am in the same frame of mind that the tone of the story requires is sometimes difficult.
This is specifically important because different novels have, as a rule, different tones. A fantasy novel is told, in my writing, in a different way than a science fiction novel, in much the same way as an academic journal article is written differently from a letter to a friend or colleague.
This is specifically a difficulty, I find, of trying to tell a story over many different sessions; it's not an issue I tend to have with official works, because they have a consistent tone that should be adhered to.
Despite the fact that classes are coming to a crux at the moment, when classes end at the beginning of April I hope to dive back into writing with a vengeance. It's an excellent creative outlet, and the best way to get better is just to do it.