30 miles r/t = 15mi per ride with an 8 hour break in between. It's good base mileage, but it's not going to condition you for a back-to-back double. I'm not saying that you won't be able to do it; but you will most likely not be comfortable with it.
Think about it in these terms: How long does your 15mi trip take you? A modest estimate is going to be 1 hour.
Now think about doing your round trip commute 3.5 times in one day, for two straight days.
If the longest distance you've ridden is 15 miles at a single time, there are many factors that you need to consider when increasing that amount by 660%. <-- Take a good look at that number. The suggested increase in distance until you reach the 100mi distance is 10 - 15%.
Hydration, caloric intake and electrolyte replacement all become concerns at the century distance.
I'm not trying to sound discouraging, but I don't want your first attempt at a century ride to be a physically painful disappointment that sours you from long distance rides in the future. If you want to know where you stand for the STP, you should ride the Flying Wheels century this weekend. Take the metric option (65mi) and see how you handle it. Then, ride a 40 - 50 miler on Sunday so you have an idea of what it feels like to do back-to-back long rides. After that, decide if nearly doubling that distance each day seems feasible.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.