Every morning I set aside one hour for meditation -- primarily just counting breaths, but occasionally abdominal noting. My overall aim is to achieve some kind of basic tranquility so I can then decide if I should move onto something else. That is, I'm doing "samatha meditation" as preparation for "vipassana meditation". (Although I have asked about that apparent dichotomy in another question.)
However, although I reserve a whole hour, at the moment I'm only doing 20 minutes (built up slowly over some weeks from 10 minutes). My current plan is to keep to that gradual build, but sometimes I wonder if I should just "go for broke" and sit for an hour at a time.
One reason I don't is the same reason I wouldn't start out by running for an hour. Instead, I'd start with a shorter period, and build it up. But I don't know if my concern is valid for meditation. There are clear physiological reasons for not building up physical exercise too quickly. Muscles and tendons need time to build strength and flexibility, so even if I felt like running for an hour, I wouldn't. I'd force myself to increase only slowly.
But does the same thing apply for "mental" exercise -- i.e. for Buddhist samatha meditation of the kind I describe? Should I "hold back" even though I'm keen to do more? Is it possible in any way to "injure" oneself by over-training when it comes to meditation.
Note, I'm specifically interested in the mental/spiritual aspects. Im pretty sure it's possible to physically overdo it by, for example, trying to sit in full lotus too long in the early stages (or, in my case, ever).