Can you run a good 800 without having previously run faster than mile PR pace?

Easily. I ran 2:14 and 5:20 off of less mileage than you, so you can easily run 2:0x if you put you mind to it.

If you're under 25, then something like this is definitely possible. Over 35? No way. In between? Flip a coin.

It depends on what type of runner you are. If you are a 4/8 or pure 8 type runner then no. At about 500m-600m in you will feel as though your body is flooded with lactic acid and you limbs will all but cease to function for the rest of the race. If you are a 800/1500 and above runner, then you will do fine. All the milers on my college team would run PRs or very close to their 800 PRs without ever running a rep longer than 200m at 800 pace, if they even did that. If you're a faster twitch, 800 or 400/800 guy then all the mileage and aerobic work is going to do very little for your 800 fitness, and fitness in general since you're not working your primary muscle fibers.