80 miles is doable based on your current weekly riding - it may not be comfortable towards the end as you'll likely get some aches from your posterior. The riding you've done so far would suggest that you're plenty fit enough to do that distance tomorrow if you wanted to, it won't be easy, but you'll look back having done it and think it wasn't so hard after all
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The biggest things to be aware of are that you always ride slower in the winter than in the warmer months and on longer rides you inevitably slow down towards the end of the ride as you get tired. Pacing is important on rides of this distance as well - you need to get used to starting slower as if you ride at you usual pace for 20miles at the start you'll burn out before you get to the end and that just makes for miserable riding.
Others have already mentioned that planning food is also important - it is - you need to assume that you will need to stop to eat at least once, even if you just take some sandwiches and snacks. Look at your route to make sure you are near shops in case you need some emergency chocolate and coke.
Lastly, especially at this time of year, you need to plan what you intend to wear - the weather is much more changeable than during summer, and if you get wet you'll get much colder, so having appropriate winter clothing is important.
These are of course things to consider for yourself, you still need to make sure you have lights - if you get a puncture you could easily lose half an hour or more if you're unfamiliar with changing an inner tube - so you will likely be riding during dusk/dark. If the weather is cloudy it gets much darker than summer towards 4pm. This is less of a problem if you are riding on urban roads as they tend to be relatively well lit however country lanes necessitate a good lighting setup.
My suggestion would be to get used to doing rides for a couple of hours from say 3pm so you get used to riding into the darkness. This will get you used to the conditions you are likely to encounter, you won't then stress as it starts to get darker. I'd also suggest leaving earlier than you might have intended - 7am would be a good start time, you'll start in darkness but as it starts to get lighter that always brings a mood boost.