I agree. I'm also a former runner who switched, though it was 25 years ago, when I was 43. I did my first century my first year on the bike, and my longest previous ride had been 12 miles. It took everything I had and exhausted me for two weeks, and I was in peak (running) condition.
Your bike will certainly go 100 miles. I know people who've done centuries on mountain bikes, cruisers, scooters and even a four-person pedal-op cart. But a century is hard--being able to ride 50 or even 75 miles is no guarantee you can do the last two hours. Train.