One problem is assimilation, where any two adjacent sounds will at least subtly, or sometimes obviously, affect each other's pronunciation. This makes describing and understanding Russian pronunciation in technical terms difficult, because there are differences in how the consonants are pronounced, but also the vowels. In fact, while it is generally described as a difference in the consonants, it is actually written in the language mostly via the vowels!
To fully understand this, you should read about palatalization (for example, on Wikipedia), including a bit of the history of Russian.
Generally speaking, palatalization is not a general part of English pronunciation, so most consonants are just not palatalized, nor are the vowels that follow. But we do have some similarities. A few consonants pronounced near the palate do get pronounce that way and therefore change the vowels near them, or sometimes palatal (high front) vowels cause consonants to palatalize via assimilation. But this isn't a relevant factor for example with [b]. There are also some interesting details like with how <u> is often pronounces "yu", like in "union".
The "short/long" distinction of English vowels is generally unrelated to this, but also tricky, and as you say, not found in Russian. Speakers of many languages have trouble with this (for example, Spanish), so it's familiar to English listeners, who can probably still understand in context. (Very rarely does it actually matter whether you say "sheet" or "shit", because those are such different meanings it's clear anyway. And yes, it's helpful that you probably would make an error by pronouncing the longer for I guess!) But the "short/long" distinction is tricky because of English spelling: these aren't actually the same sounds, just shorter/longer. They're different qualities of vowels. English spelling was standardized about 500 years ago, and around that same time the Great Vowel Shift was affecting the pronunciation of long vowels, but not short vowels. So the short vowels mostly remained in their original positions (you can see this by comparing their pronunciation to Latin or various other languages, or even transliterated Russian), but the long vowels are now just different sounds, about "one place" away within the vowel space. But then we have pairs like "ee" [i] vs "i" [ɪ], which you identified, but which are not generally called "short/long" pairs by English speakers, although linguists know this of course.