Originally Posted by
dwmckee
... and have ridden a few centuries. I am ready to move up to 200k + distance rides this year...
If you are comfortable doing the centuries (in a reasonable amount of time), then a 200k (125 miles) is just another 2 hours (about) of riding. You might not need to do anything different. As a rough guide, your projected pace for the 125 should allow you to do the ride during daylight hours.
If you are talking about 200k randonees/brevets, then you might be looking at it as a step towards longer rides (like 300k's). In that case, you might need to consider lighting requirements (many randonees/brevets start before daylight) and being able to navigate without road-markings (fairly common for organized centuries but not for randonees/brevets).