Lots of people manage it, therefore it
is realistic. Let's face it -
you are already over 3/4 of the way there and it isn't even summer yet!
If you want a more useful comment - I rode from Hebden Bridge to Coventry in 2007 over some reall lumpy bits of West Yorkshire and Derbyshire. I was 20 pounds over my best cycling weight and I had (small) panniers on my bike. I was reasonably fit, but not at my best level.
I wasn't in a hurry. Total distance 227 km (141 miles), 4,000 m of climbing (13,000 ft), time taken - 12 hours 45 minutes. Okay, it wasn't 200 miles, but it was harder than a 200 mile ride without hills would have been.
In fact, when I got to Coventry, I could easily have ridden another 59 miles around the flatter Midlands countryside if I'd had the daylight hours and inclination to do it.
A flattish mid-summer 200 miler would be relatively straight-forward for any cyclist on a comfortable bike who did enough miles beforehand to build up to it. It took me about 18 months to gradually build up to that kind of level and I'm nothing special.
I don't aspire to being a super-long distance cyclist, but I think a 300 km audax and a 200 mile ride are probably things I will do in the future. I may even do a 400 km audax, but I think that would be my limit. I wouldn't want to get into sleep deprivation or kipping in fields, bus shelters or church halls!