I've been riding for 9 years, and a 50 mile ride can still leave me a little tight and sore the next day; obviously not nearly the pain it left you in, but I still know I rode the bike the day before. Going from nil to 50 miles like that is probably the biggest problem outside of proper bike fitment. I think signing up for a 100 mile ride with almost no time on a bike (in my opinion, you should wait until next spring to ride that far) may be a really bad idea.