I have just completed my first SR (including my first 400 and my first 600).
I did the 600 first and the 400 a month later.
IN MY OPINION the 600 was harder. It was harder because, amazingly, it was 200km longer, requiring approximately 11 hours more pedalling and sitting on a, by that time, pretty uncomfortable bike for another 11 hours.
I didn't have any issues pedalling for so long, I mean I never thought "I can't pedal another kilometre", my problem was (and always will be it seems) a recurring shoulder cramp. I took it slow and steady and eventually got to the end. If someone had offered me £1000 to cycle another 200km at 15km/hr then I probably could have kept plodding along (of course your mind gets fixated on finishing in a specific distance so you tend to JUST be able to get to the end whether it's 200, 300, 400 or 600). You may feel knackered at the end of a 200 but during a 600 you find that 200km comes and goes as if it were 50km, lots of this is mind-games.
So, depending on how hilly the rides you choose will be, I reckon that if you can keep pedalling around a 300 then you can do an SR. Just expect to be drained on the final 100km (I didn't sleep on either 400 or 600) and make sure you save any sprinting and honking up steep hills for the final 10km rather than the first 10km (ie. save your energy, you may need it later)
Go for it, what's the worst that can happen?
(Someone will now come up with a list of the worst things that have happened I expect)