Yup, you can do this. I did my first century a few weeks ago on a 31-year-old 5-speed mixte with "sit up and beg" handlebars. I didn't train - other than my 12-15 mile commute every day and the odd 35-mile ride on a Sunday. Lots of people told me "if you're fit enough to ride 35-40 miles comfortably (and not suffer the next day), then managing anything over that is totally about comfort".
Good luck with the ride! Hope it'll be the first of many!
Rebecca
Riley - custom 2014 Enigma Etape
Bridget - 2010 Surly Cross Check
Lorelei - 1979 Puch Princess mixte
Astrid - 2014 Viking Bromley singlespeed mixte
Lucy - bespoke 2012 Brompton S1L
Visit my blog: velovoice.blogspot.co.uk