Grammar girl has a highly readable explanation of what a run-on sentence is and how to fix one depending on the writing style you want to achieve: http://grammar.quickanddirtytips.com/run-on-sentences.aspx - also available as a podcast.
Here is a fun example of a really long sentence, though not a run-on since it is all connected phrases:
Making the best use of the morning commute is a challenge, when kids are still sleepy and not ready for anything too challenging. One possibility is to play an audio snippet. To play in the car without wifi, save the file ahead of time by right-clicking on the link to the .mp3 and choosing 'Save link as...' or similar.
http://writersalmanac.publicradio.org/podcast/ has free podcasts about 'this day in history' with Garrison Keillor reading
Mark Twain snippets have been recorded and some of the funny bits are available here: