good on you!

if you can do 50 miles you can do a metric century (100km or 62 miles) so roll it on!
i recently did my first imperial century having done a couple or so metrics by accident and was surprised by just how easy it was considering i hadn't done more than my 15 mile commute for a month or so
just split it up into suitable chunks, for me it was 20 miles. so i rode 20 miles pulled over pretty much anywhere had an apple and/or snack bar and a sandwich at lunch and a sip to drink and stretched my shoulders which i find tense up if i'm not careful and simply get on with it

i say go for it, set your self a target say first metric century before the end of this year and then first imperial century before end of 2015. should be by all means possible
Cheers Ed