I've done a century once but this year's charity ride is 4 centuries (160km) in 4 days (640km, except for those who like torture, they're doing up to 830km in the 4 days). We will be about 20 riders, will have a SAG with food and water stops every 30-40km and the route is relatively flat with a total of about 1,900m (6000ft) climbing over the 640km (I haven't seen the route for the 830km but on the 3rd and longest day, 800m/2500ft of climbing has been added, just for them).
I'm a little scared of not being able to do the 4xCenturies but I'm going to try and I do have until mid-September when the ride will take place. I'm also 57 and although I commute most days of the week (30-60km/day depending on the route I decide to take) from April to September, I don't do a lot of distance in a day. It's the fear talking in me right now, trying to prepare me for failure but I have to overcome that.