You can do it now. Two things to consider:
Do it on a day when you have a tailwind. It will be much easier if you do
Either take plenty of food or suss out in advance where you can buy food, such as Co-op. Even after a decent breakfast of you're cycling for 7-8 hours you are going to need plenty to eat. Drinks likewise but it's cold and you aren't going super fast so you can top up on route. I often ask folk in their gardens for a top up and I've never been refused
You sound as if you are doing quite well in terms of weekly mileage, and are comfortable on your bike. You seem to be more determined than myself in getting out regularly despite the weather recently. A drawback to everything at present is the Coronavirus lockdown, which means that some shops and facilities are closed. This not only affects food (though you can carry that with you) but also the availability of toilet breaks -quite important when you are planning to be out for longer than your usual rides, and particularly all day. Supermarkets or garages with shops are handy in this respect, though I would recommend having a good lock, and if you can park near the entrance, preferably under the gaze of staff, all the better. If the worst comes to the worst, carrying some loo roll can be a life saver if you get the call in the middle of nowhere.
If you can get near 50 miles on a ride, 80 miles is not such a big jump. To load things further in your favour, adding to what other posters have said, if you can make a really early start after a good breakfast, by the time you have a snack at your normal breakfast time, it will be a big psychological boost when you consider that you'll already have taken a chunk out of your mileage for the day. I am by no means an early bird but if I wanted to cover a distance at this time of year, that is what I would do. It is easier to set out in the dark on familiar roads, as it only gets lighter, and towards the end of the day when perhaps you are a bit slower and tired you won't have to do it in the dark. 10 miles before breakfast is worth 20 after lunch! A still calm day or one with a tailwind is a bonus, and it helps to plan your route in advance. There are still good days to be had at this time of year, although it is hard to believe after the last couple of weeks' weather.
Whether you decide to do it in the next few weeks or next year, good luck. I still remember the buzz I had when I first did an 80 mile day, and that was 50 or more years ago in hilly Wales.