FBinNY 

IME, it's relatively easy to add half again to any distance you're able to ride comfortably, and there won't be much falloff of speed toward the end. So I suggest you use this half again guide to push your limit once or twice a week until it too can be ridden back to back or even daily, then push beyond again.

In short the ONLY way to increase your range is to ride farther and longer, then doing it until it becomes "normal".

BTW- it's not only distance, depending on geography and rider condition, the limiters can be how many hills of what height you can digest, the total miles, the saddle time, miles at speed, (ie. you can ride 20 miles, nut not at a 16mph pace) and even temperature. I know someone who's feet (the soles) get tired and crampy long before anything becomes a limiting factor.

BTW- because of deadlines, the best time to increase range is on the way home. That lets you get to work in good shape, and not fret time or speed on the way home. If you bite off too much, stop and rest, then continue. I do this daily, taking the 6 mile route going in, and riding anything up to about 30 miles on the way home depending on mood and weather.