Long instead of short vowels

Hi,
I learned (and recognized) that in certain words (stressed syllables, in reality), the vowel is longer and on two steps of intonation when the syllable coda is voiced, otherwise it's short. In other words, I am just talking about the difference you can hear between GOT and GOD, for example. The second, "god", ends in a voiced consonant, and the AW vowel is longer, on two steps.

Now, I've always thought that "rule" had several exceptions. In fact, it seems to me I often hear long vowels when short vowels are expected. I tend to use long vowels in "this" and sometimes in "but", when followed by a pause, and that make it sound almost like "bud"... "I know I was wrong, bud... I think you were wrong too".

Now, what do you think about long vowels instead of short ones, when the syllable ends in an unvoiced sound? Here is an example, where COP is pronounced with a long vowel, or at least so it seems to me. If I didn't hear the final P clearly, I would probably take it to be COB. No need to watch all the video... The first sentence she says is the example:
Hi! Today we're gonna be talking about the difference between COP and CUP...

Hi,
there are several different ways to classify vowels (also tense vs lax, for example). What I call vowel length is what is considered in linguistics, I think. It's the difference you hear in General American English between the vowel in GOT and the vowel in GOD, which is the same but in "god" it is longer, not clipped or anything.
Now, I think that happens in stressed syllables, but even in that case I realized there seems to be several exceptions. In other words, I think sometimes (or often?) American native speakers use long vowels instead of the short ones. One example is in the video: at the beginning, she says COP with a long vowel, the one you would usually use in COB instead. If she hadn't released the final P in that COP, like it's often done, I would have understood COB.
Can anyone comment and tell me more about this? Thanks.

Hi,
yes, I am talking about American English. And welcome to the forum. We don't have many Australians here... hmm, maybe you are the only one! So double welcome.