The trick to riding any distance is to build up to it
So this time of year get used to riding 100km
If you can get out on the bike at all during the week, even for an hour, do it.
So the idea is to do as much mileage per week as possible, with 100km as your basic easy to do distance. If you can get to the stage where 100km is easy-ish then try going faster. Try something like doing the second half of the ride at the highest pace possible.
When your first 200km is due try and do a long ride a couple of weekends before, maybe 150km. Week before the 150km do a 100km. You should be able to cope with the 100km as you have been trying to do a few over the winter. Week before the 200km take it easy, don't ride, rest.
So I suggest a 4 week build up: weekend 1, 100km, weekend 2, 150km, weekend 3, a short light ride, week before the 200km no riding, then the 200km at the weekend
Some people say that the "short light ride" should actually be something like a warm up then an hour of hill repeats. So not "light" as such, just short.
A thing I've noticed with beginner audax riders they don't pick their battles. Don't do events in January or when the weather is bloody awful. It's just making another obstacle. Wait until later in the year