Good question!
With the increasing popularity of Sportive's up and down the country and even Grand Fondo's in central Europe preparation is key.
Too often even on the shorter charity rides such as London to Brighton people start with gusto only to fade before Turners Hill with half the course including Ditchling Beacon still to ride. A daunting prospect to the inexperienced cyclist which can turn into a miserable experience instead of a joyful occasion without a little simple training.
Its all very easy to work out. Take your sportive length - kilometres sound impressive but we measure in miles here! Estimate [realistically now!] your average speed taking into account whether its flat, hilly or a mixture. Trust me, on a lumpy course you'll be down to around 14 -15mph - nothing to be ashamed of you're still twice as fit as the average 'couch potato'!
You will be capable of riding 3 times the length of your average ride. Typically if you train 1 1/2 hours 4 times per week = 6 hours you'll be able to ride 4 1/2 hours around 60 -70 miles.
Likewise 90 miles at 15mph will be a 6 hour ride so you'll need to train 2 hours 5 -6 times per week.
Riding with a group of likeminded friends or with club will always help. Using the 'talk test' when riding will give the correct intensity, you should be able to just talk in short bursts - don't worry about power output/HR, just enjoy the ride. If you record it look when you get home or let me analyse it - if I coach you!
That's the basics if you really want to succeed contact me: coach@merlincyclecoaching.com