My work schedule and kid make it not feasible for me to ride outdoors during the week. I have two VERY high quality (90 minutes each) spin classes during the week instead, and a long ride on weekends. This has generally been my practice for 6 - 7 years (though the spin classes have been much better in the last 3 years). I have been very successful doing this. And I train for double centuries (200 miles plus in a day, not back to back centuries).

And - if you can do 50 miles, you can do 100. Really. Just work the mileage up over the weeks before your century.

And have fun.