I agree with those tips Matt. There's no time limit for defenses, so conquer planets and build up armies and defenses.
I find it good to build a huge space/ground fleet for conquering planets, and leaving small defenses with space stations at my controlled planets. A big space station and one of each ship available will stop any enemy space invasions, and a few troops and vehicles on the surface, along with stuff like turbolasers, should stop any small land invasion that takes place.
Just take your time when it comes to campaign, it's all worth it at the end.