On the Twelfth Day of Christmas My True Love Sent to Me...

...Twelve Drummers Drumming! I imagine the sound as urgent and climactic, rallying the listener (and in this case the reader) onwards. Drums, whether at a gig or on the field of battle, are there to energise and inspire, setting the pace for a song or a march, and pace is something extraordinarily important when you are writing fiction.
Whereas tension is linked to suspense and has to do with the facts of your narrative - discovering who  murderer is, or whether true love will win through - pace has more to do with the way in which you relate these facts.
Uniformity is the enemy of pace. There should be variety within the structure of your novel. This applies from the macro to the micro: try and make sure that your chapters, your paragraphs, your sentences, and even your individual phrases are of different lengths so that your work doesn't become predictable. If you are writing about something dramatic, keep things short; if you are in more laid-back mood you can afford to meander (but not too much). Just keeping an eye on this simple technicality will have an enormous impact on the readability of your story.
Another way of manipulating the pace of a piece of fiction is through contrast. Just as you should go slow, slow, quick, quick, slow or long, long, short, short, long in structural terms, it's important to contrast the types of scene you are writing. If you have ratcheted up the emotion during a particular sequence of your book, give your reader a break and introduce something lighter. Think Hamlet and the gravedigger scene.
Pace has elements of texture and tension and contrast within it. It's what gives your writing dynamism. It's an essential ingredient in any good book and you should always be conscious of it when you are at work.

Moving from Darkness into LIght

I wish!

It's pretty dark here and  lots of that sleety stuff  is drizzling down outside in a can't be bothered kind of way, as if the very idea of getting any momentum going so early in January is out of the question, and I'm feeling so enervated I can hardly stay vertical, just looking at it.

But I am vertical, sitting at my desk, trying to add some momentum to my thoughts, thinking about what gets you going and keeps you going. In that vein, it seems to me that it isn't just the story itself which sustains a book, but the way that it is told, and that a crucial way of adding a sense of internal drama (and momentum) to your work is through contrast.

Think of all the different ways that you can achieve contrast in what you are writing : you can change the depth of focus from close-up to longshot, you can move from interior to exterior, or from action to contemplation.  You can change from narrative to dialogue. You can move from the subplot to the main plot and back again, or mix something serious with some light relief. Anything, rather then get stuck in a linear this happened and then this happened rut.

See if it helps you move from darkness to light in what you are writing at the moment, from snow to thaw, from winter to spring....