If you don’t mind road rides then you could enter some audaxes. Visit Calendar Events| Audax UK - The Long Distance Cyclists' Association and look at events near you.
For instance there’s this 300km event in Devon this weekend. Old Roads 300 Event Details| Audax UK - The Long Distance Cyclists' Association. It full but of plenty of other events throughout the year.
Audax UK events go all the way up to 1400km this year and are all self sufficient and involve checkpoints and brevet cards.
You could start with a 100,150km event to get used to the format. Then select some 200km events to up your distance.
Most of us jump straight from 200km to 300km. Once you’ve worked out your setup for 200km, the leap to 300km isn’t so much. You just need to think a bit more about the wider range of temperatures and that you may need to do some riding in the dark.
There’s also RRTY where you need to ride a minimum of 200km in 12 consecutive months right through winter. Harder than it sounds.
I tend to do the RRTY 200km min a month to keep my fitness and motivation going year round.