You're OK. Your average speed and the fact you can do 75 miles tells me you can do 100. Assuming it's an organized charity-ride-type century, take your time and have fun. It's a ride not a race. Find fun folks to ride with. Spend time at the rest stops. Eat a good lunch. Help someone fix a flat. Take your time.
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Truth is stranger than reality.
'96 Giant ATX 760 MTB
'01 Bianchi Eros
'05 Giant OCR Llimited Carbon Fiber + upgrades