Straight away, the answer is do not use an iPod Classic when running!
All models of the
Classic have a hard drive inside - that is, an arm (with a read head) moving across the disc to find the music. When you run you are shaking the iPod which will make it more difficult for the read head to stay in the correct position. When running, it won't take long therefore, before the read head cannot maintain the correct position and the sound skips. Although the occasional "run for a bus" is probably okay, in my opinion, exercise running will put excessive strain on the moving parts of the iPod and lead to early failure.
If you want to use your iPod while running, it's far wiser to use a flash drive model. The flash drive has no moving parts to get shaken about. Check this page;
http://support.apple.com/kb/HT1353 to identify the flash drive models. Look below the table/chart for information about each model. Basically, if it's 16GB or less, then it's a flash drive.
If you choose the iPod nano 4th generation you will need to remember to disable (turn off) the
shake feature, which shuffles to another track when you shake the iPod - i.e. when running.
Once you've decided upon a model, there are various armbands available, although I've not checked them out so I'm unable to recommend any of them.
Good luck.
