After a summer of riding mostly weekends, and shorter 25 mile rides (most peoples winter), I just ride 25's after work, and start bumping my weekend rides up a bit longer, until I'm happy. They usually end up around 50-60 milers, and that'll get me to 70+ just fine. Nothing complicated, and no training formulas to follow.

​More than 70ish miles and my brain gets bored with riding anyway.