The general recommendation for increasing distance (for distances under 200km) is that you shouldn't push more than about 10-15% on your next consecutive longest ride. Over 200km and up to 400km, you're really just managing food intake. For 400km+ (in a single ride) and you're talking about needing to manage sleep into your schedule.
Since you've already determined that sleep will be a part of this 250 miles (400km), then you're looking at food management (easy) and base mileage vs. this upcoming mileage.

Current longest day = 70 miles, last year sometime. 45 miles last month.
New longest day = 100 miles. 43% increase over your longest day last year, and 55% over your longest day this year. That's a huge increase in mileage, and you're planning on doing that back to back after already going 5 miles longer than your current longest ride.

Again, I'm not saying you can't. I know people whose longest day has been 30 miles, and they go and ride the Seattle-to-Portland in 2 days (204 miles, back to back centuries.) A guy here at work rode a 185 miler 2-day last year with almost no time in the saddle for training. It can be done. I'm just suggesting that it might be more easily accomplished with more base miles and some longer training rides under your belt.