I did my first 300K three weeks after my first 200K randonnee. I had ridden a number of centuries and a 200 km ride at my own pace before that, so the 200K wasn't a big deal ... but I'd never ridden more than that, so the 300K was a bigger deal.
I'd have to dig out my old records, but I think I did my usual commute in between the rides, plus a 100 km ride and an 80 km ride on the weekends in between.
And yes ... you do have to pay a bit more attention to nutrition. Also, it's a good idea to travel prepared for flat tires, darkness, etc.