Originally Posted by jrickards

I'd heard that mimicking the "Asian crouch" (not being racist, just an observation of people who are comfortable sitting on their haunches), even if you have to hold onto something to keep balance, is good for the back, it does me good (I need to hold something for balance). Sit in that position for about 30 seconds, especially after the ride, and your back will stretch out.

I know from my limited weight lifting experience that if you work too hard too early, you tend to develop bad form in the effort to complete the workout and this probably applies to cycling too; if you go on long rides too early, you can mess up your form in the effort to complete the distance, your muscles may not be strong enough. If you're commuting the shortest route, you don't have much choice about distance but it makes it that much more important that you concentrate on form while riding. @wphamilton said it best about form. Core strengthening exercises (core includes lower back as well as abs) also will help.