I've been doing 20-30 min anapana sits for a long time but that started to feel to short, like when the timer rang I would just have started to get in a groove. Now after having arrived from a trip where I got only one sit a day, I've been trying out 40-45 min sit more often and at the end I really have to fight the urge to rise and do something else since my posture feels so fucked up. Should I use willpower to continue sitting when it feels like that? I've always tried to use as little willpower as possible, in the sense of Ajahn Brahm (and John Lennon haha). 

I know you've probably heard this before, but go back to the object of your meditation (using anapanasati, that would be the breath) and bring the mind back to that object for further concentration. And keep bringing it back as many times as necessary during the duration of your sit. That is, unless you are doing vipassana (insight) meditation. Then you can refocus upon the subject of your insight contemplation and explore it.

In performing the above with regard to anapanasati, you will be developing your willpower over the tendencies of the mind to want to do something else. It is all grist for the mill, so to speak, in retraining the mind to hold still so that you can develop your insight contemplations to the degree that they begin producing fruitful insights about the Dhamma.

Concentration isn't the only way to remove a distraction.  You could meditate in a shallow pool of hungry piranhas and try to remove aversion with concentration, but that isn't exactly a gradual training.  Find a way to be as comfortable as possible, both emotionally and physically, and it will be easier to concentrate.  You don't even need to sit still, really.  If scratching an itch means it leaves your mind faster, just do that.

On the other hand, if you're sitting on a comfortable sofa bouncing your leg and clawing at every physical sensation that bothers you, you're probably just suffering from boredom and need to sit through it for a bit.  Samadhi is about skillfully fabricating a pleasant state.  You are essentially compensating for a lack of acceptance with concentration.  As you train the mind to be more equanimous with unpleasant things, you need less concentration to remain in a pleasant state.

contentment = acceptance + concentration

If either one is at 100%, then you're in the perfect state.  A sutta arahant is 100% acceptance, nirvikalpa samadhi is 100% concentration.

Thanks guys.

@Not Tao
that last statement about sutta arahant vs hindu enlightenment was very interesting. Cpuld you elaborate?

@Ian And
yes I know, go back over and over again, that's kind of what I do. But then, what's the point with these instructions? 
http://www.accesstoinsight.org/tipitaka/mn/mn.020.than.html
Here we go again ;)

I think the main difference is that nirvikalpa samadhi is just one state among many, while acceptance encompasses all states. I don't think Hindu and Buddhist enlightenment are different, though, just different ways of describing something mixed with dogma about what it "is". Nirvikalpa is the culmination of Raja Yoga but not Jnana Yoga - which is "knowledge" yoga. In Yoga, Nirvikalpa is being with god, while Jnana is understanding the nature of god (or Self as god).  The culmination of Jnana yoga is "effortless samadhi" which I can't remember the name for (why do Indian people have to give everything a 10 syllable name?! Haha...)  EDIT: It's "Sahaja Samadhi."

You might find this interesting: http://sped2work.tripod.com/samadhi.html

Thanks! We were taught in school though, that Jnana Yoga is not one single yoga tradition but contains most yoga methods other than Bhakti Yoga and Karma Yoga. 

If you have to force yourself to sit, you have not figured out how to meditate yet.

Go read the _formal instructions on insight practice_ in MCBT.
Pay attention to the tiny movements and vibrations, feelings throughout the body in as much detail as possible, as you repeat a mantra synchronized with the breath followed by (silent) 5-8 breaths. The mantra and breath count is to keep your mind from wandering. Breathe in a way that brings you pleasure.