I have found that patience is a virtue in this game. Don't go into battles unless you can win them with low casualties, even if that means sacrificing honor. This is especially the case at the beginning where you will have units gifted, won, or from heroes that you cannot easily replace (e.g. Griffins for humans, ghosts and vamps for necros, etc). You should guard those carefully and try to only use them in cases where you will not lose any, such as overrunning the last stack.
Shoot for the level 2 recruiting buildings and you can upgrade your existing level 1 unites by hitting the upgrade button on the recruit screen for that unit. For necros, skeletons upgrade to skeleton archers, which is really really useful. Better yet, it's free. Once you get zombies, convert all your skeletons to archers and you should have quite a force of shooters.
Since the pace is so slow, I haven't yet found a reason to take anything but the maximum amount of time to construct a building or a mine. It takes longer, sure, but you've got time to kill. The savings in gold and resources is great, though. At max time, a building only costs 80% of the normal gold. You'll find that you will be out of gold most of the time anyhow, so anything you can do to conserve it is worth the time.