Just do it!
It's all in the head.
+1
I dithered about last year riding only 100s, 200s (some flat, some hilly) and a single (very flat) 300 (although riding to/from some of the 200s pushed some of them up to about 270km).
I wish I'd just gone straight for an SR. Now that I've done it I can honestly say that the old adage of:
If you can do a 200, you can do a 300.
If you can do a 300, you can do a 400.
If you can do a 400, you can do a 600.
is true.
I'd just temper it to:-
If you can do a 200 without too much stress and with a bit of time to spare, you can do a 300.
If you struggle round a 200 and get in just within time then expect an extra 7 hours of the same on a 300, but you can still do it.
If you struggle around a 300 then you'll find a 400 even tougher because of sleep dep issues.
And any ride can turn into a horror if the weather isn't kind, or you're suffering for other reasons (dodgy stomach, sleep debt, etc).