Do a web search on century training regimens (or even something like a marathon run training regimen). They will all suggest a gradual increase in a weekly long distance ride as well as in the total weekly mileage.
You can go straight from 50 to 100 miles (ie. 3 hour ride to a 6 hour ride) but it isn't very wise, probably won't be very fun and you may even be risking injury. I would build up a little slower (maybe something like 10-15% per week).