One of the most interesting experiences I had on this particular meditation was that I gained a new feeling/understanding of time. In my current meditation I simply meditate for as long as "feels right", but for those 40 days we used a timer. On most days we set it for 11 minutes. During those eleven minutes there was nothing I could do to change how long it would take. I couldn't breath faster or breath slower. I couldn't chant louder. I couldn't think harder or less hard. I had to completely surrender to the passage of 11 minutes. And some days it felt like a long time and some days it felt short, and knowing empirically that it was exactly 11 minutes every time really opened my eyes to how mental state affects the experience of time but not its passage.
And really this is the only thing we can ever do to time. But when I'm not being as mindful I certainly try to change it! Maybe I rush whatever I'm doing so I can get to the "good stuff" faster, and in rushing I make mistakes and end up slowing down. Maybe I spend all my thoughts on the next thing I'm going to do instead of on what I am doing. Maybe I move slowly towards and unpleasant task, only prolonging the experience of unpleasantness. Maybe I think that if I only go faster I can get that 2 hour task done in 1 hour. I rarely stop fighting time. I rarely stop to simply let time be the length that it is.
Spending 40 days experiencing the length of 11 minutes with nothing else to distract me was an important reminder about how time works. It turns out that you don't actually have to DO anything in order to make it through 11 minutes of sitting, inevitably you will. And there's pretty much always time in the day to carve out 11 minutes to be mindful of that fact.