I roam in a red/yellow combo like this:
http://www.lotrottp.lotrostatus.com/l.php?c=Wh, except with the additional five trait points, I have AC Lightning storm for the +5% armor (not much, but it's better than my other options in the line) and +.75% stun duration. Obviously blue or yellow line would be optimal against wargs (at least speed wise) but my tips will be coming from what I would recommend play in that line.
First off, don't take it hard that you are unprepared with SI- if you are riding any longer than 20 seconds, SI will be gone anyway. Same goes with Water-lore, which I hardly ever pre-stack in landscape anyways (makes you look like a scared nub

).
Step 1: You are knocked off your horse, I suggest using your escape skill or brand, do NOT pot it, as pots are totally bugged on the timng (as Til sad).
Step 2: Pop Call To Valar. It's a five minute cooldown skill, but better to use it than be sorry you didn't- you don't donate unused skills to charity. Immediately turn to face the warg (fast turning is critical with wargs), and Pop Water Lore.
Step 3: Use Fire Lore, and follow it with a Burning embers DoT. That first Burning Embers DoT is critical- if you do not slow the warg, you are dead meat.
Step 4: At this point I usually start to kite. The warg will be slower than you, so he shouldn't get much positional damage on you. While kiting, get SOP Command and Wizard's Fire up.
Step 5: All good wargs will prepot before jumping a LM. If they do not have that terrible little blue circle of pre-potting on their vitals, then immediately use storm lore, and the fight will go much smoother. If they have pre-potted, at this point in the fight, that Immunity will have worn off. Make a sharp turn in your kiting- aim directly at the warg and use staff strike for the stun.
Step 6: Continue moving past the warg to get a little bit of distance. Turn and apply another Burning embers, blinding flash, etc.
That's basically it. Sprint can be another tough thing to handle with, but the most important things to remember is to keep fire lore up, get that burning embers initial slow on it at all costs, avoid positional damage at all costs, and use your dang interrupt immunity.
Hope that helps. As Til said, a good warg is a demon to handle. Keep fear pots for silences, disease pots for fleas, and use SI if it's off cooldown to cure silences as well.