In reply to jules699:
Most zero-to-hero cycling training plans I'm aware of generally work on a 12 week timescale.
There will be some shorter training plans, but be aware that a 5 week plan doesn't give time for a base build, followed by upping your game and ending with a consolidation & final tapering period. There is an injury risk with rushing stuff.
A 5 week plan is more likely to be a fine tuning plan for someone that either already has a base or has let optimistic objectives.
All really depends upon where you are now?
How far could you cycle next weekend?
Would 50 miles leave you in distress & knackered for 2 weeks?
How about riding 60 miles in 3 weeks time? If you could do that and take on any lessons learned about feeding, comfort and bike setup then you could probably do the 100. Not necessarily fast, not necessarily pretty, but you'd probably get around.
Do some shorter easier rides the week before.
This might be interesting reading for you:
http://roadcyclinguk.com/riding/your-first-sportive-part-1-going-for-centur...