I think the 100 mile/200k distance is possible for anyone with a reasonable level of fitness who is willing to put in the mileage to build the base you need to go that long. I think you need to be able to ride 6 hours without dying. For me, the 60-125 mile distances were the hardest. That is when I had to work hard to build up my muscle strength in my back and neck, and toughen up my undercarriage, and really nail down nutrition/electrolytes. I think another important issue is just having a love of cycling for long distances- if you don't enjoy it to begin with, it's hard to put in the hours necessary to build up your mileage base. _After_ you can comfortably ride 100 miles, not just survive it, then the longer distances come relatively easy, compared to when you were working up to 100. And after 200 miles, as long as you are as comfortable as possible, and can maintain calories going in, and keep the right electrolyte balance for how much you sweat, then all it really takes is mental toughness to just stay on the bike.

My time frame was:

July: Rode MTB with road tires, base of 35 miles.
Last week of August: Got road bike. Had a base of about 50 miles.
September: 50 mile ride in mountains.
October: 80-90 miles on paved trails. First Century.
November: Second Century
December: Maintained base with 60-80 mile weekly rides, with shorter weekday rides.
January: 200k (no weekday riding at this point, most weekend rides 30-40 miles)
February: Century, personal 112 mile ride, 300k
March: 400k
April: 600k and back-to-back Centuries

Planned rest of season:
April: Century
May: Mountain Century, Wickham Park (running) Marathon, hopefully sprint triathlon series over the summer

Maintain base during summer by riding 70-80 miles Saturday, half that Sunday, 45 minutes to two hours on weeknights as time allows, with two rest days.

September: Back to Six Gap (100 miles over six mountain gaps) with hopes of doing 100 miles rather than 50.