I bumped up to my first century pretty quickly. Did a half in the mountains around Charlottesville, VA one weekend, then did a three-day ride from NYC to DC about 2-3 weeks later. 82 miles the first day, 127(!) the second and 69 the third. I survived it, but I don't know if I'd recommend your first metric century and real century on consecutive days! It took a while for my butt, neck and wrists to recover. Of course, I also did it on a mountain bike which is definitely not as comfortable for that kind of distance.

Looking forward to my first time reaching these milestones on my new road bike!