Make sure at least the High/Low stops are set correctly on the derailleur so that you can't physically toss the chain off into the spokes, or move the rear derailleur into the spokes.

As far as making it harder to pedal, are you sure you're getting into all the gears?

The easiest gear, of course, is small front, large rear.

Out of adjustment could cause some premature wear, but shouldn't be a major issue for friction.

Poor fitness after the time off?