I'll add one thing to all the excellent advice here: learn to use your gears. Go to a deserted parking lot on a Sunday morning and practice, practice, practice shifting up and down from low to high.
Nothing makes me cringe quite like seeing some poor newbie grinding away at far too big a gear, shoulders heaving from side to side with the effort, kneecaps no doubt being ground to dust. Sometimes, if I suggest to such a person that it would be easier in a lower gear, they'll admit they don't know how use them. Perhaps all those cogs and chainrings intimidate them, but it isn't rocket science, it's just cables and pulleys, and you won't break anything unless your bike is in really decrepit shape.
If your route is very hilly, a compact or triple crankset can make the trip more enjoyable. Or, ask your lbs about lowering your gears (if needed) by installing a wider range freewheel and possibly a long-cage derailleur as well.