Not to put too fine a point on it but a two step process to being considered reliable is:
1/ Say what you're gonna do and
2/ Do what you say you're gonna do
So the old salesman's trick of "under-promise and over deliver" still holds true. I think these days it's also known as "managing expectations".
Moral of the story is to resist being coerced into making promises that will be hard to keep. You're only delaying the pain and setting the scene for a bad reputation.
I - too - should learn to say "no" more often.