I'm pretty terrible at RTS games, so this is troubling. I'm in The Shiel, and I have two bases and about six different places that can get attacked, a troop cap of 30, four heroes and 19 soldiers.
Is there a particular strategy to attempt in situations like this? Do I zerg rush the most irritating enemy base first, come back and clean out any invaders, thus limiting the number of fronts I have to defend?
Or do I send heroes or soldiers "commando style" while I use my main force to defend these several sectors?
My ability to think on my feet is being tested constantly. I had everything going well until I started getting flanked from behind, and now my human base's defense force, my four heroes, is being overwhelmed. Not challenged, but incapable of defending everyone.
Timing appears to be everything.

Timing is important - also though keep an eye out (especially early game) for enemy scouts - you get a noise and a yellow circle appearing on the minimap if they notice you - you then have to kill them quick before they get back to their spawning base. If they do then the enemy has found you and willl start to send more serious waves of troops at you.
Scouts tend to partrol the same routs so get to know them and position your troops to take them out - sometimes letting the scout get closer is better as that gives you more attacking time.
As for dealing with maps there are generally a few smaller enemy bases on most maps - backed up with much larger main targets. My advice is to identify and then take out the smaller camps first - sometimes your hero and accompanying heros can do this early in the game - but don't take too many risks. Last thing you want is to start losing your main hero early in the game and having an activly attacking spawning point against you.
Generally a backup of troops is needed and - on some maps - splitting up might be an option - at 30 I would tend to keep them together.
Also always try to complete a map without leaving it to another land - or losing it totally, Always save when you enter a new land and keep that save unchanged so that if your plans fall apart you can reload from the start of the map. This is important as when you leave an area and return resources are not always regenerated (I think some do - but it might be linked to time away or some such) essentially that means if you harvest everything and then get beaten and try to return its going to be even harder since your working with reduced max income.

Ahh, I was curious as to what happened if I left. It seems like stone and iron regenerates very slowly, but I'm not sure about trees.
I think somewhere along the line I alerted a base- one of my artisans, picking berries I presume, went on a huge trek across the map and alerted practically everyone. I didn't think they'd journey that far!

I just played the map of the Shiel myself, and that map is definitely good at throwing enemies at you from all directions. A couple towers backed up by around 6 units can hold of enemy troops pretty well without any attention on your part, so set up this kind of defensive arrangement wherever the enemy is choosing to attack from so that you don't have to spend your attention constantly micromanaging your defense. Also make sure you have a Food Store for both elves and humans so that your units with regenerate.
Your group of heroes plus a few additional troops can easily take out several small camps nearby, which will take a bit of the pressure off. The main camp that will give you trouble in the Shiel is to the northeast, since as time goes on it will start sending a near constant stream of medium strength orcs at you. You'll also need a decent sized force to take out this camp, but try to do so as soon as you have enough troops, since once you deal with it everything becomes real easy real quick.
Also, as overread mentioned, watch for scouts and take them out early in the game, as this can hold off the pressure for at least a little bit until you have time to put up some defenses and build up your forces a bit. Finally, when playing with elves you need to handle your resource gathering a bit differently, as they mainly only use wood, which ends up burning through your wood supply extremely fast. Usually you want to set up a Forrester along with two Woodcutters Huts (staffed with 10 workers) as well as a Sawmill. While this main seem like overkill anything less tends to leave you short on wood in no time.

i'm playing this on hard.
i use an heavy combat arts avatar
so its good at tanking.
i usually start harvesting 40 wood.
then i immediatly build the woodcutters and start harvesting the berries with a food store.
i try to limit the space between all the buildings so i don't have to run my ass off.
loo for a nice space to plant a forrester and leave it there for about 5 minutes. then you can build the master woodcutter or 2 normal woodcutters.
i've planted towers everywhere. so when i left i had no problems scouting the map.