I as well ran XC and track competatively...
Even returned to it in 2000 and was competative in San Diego city races.. nothing spectacular but I could run a sub 20:00 5K... (in high school I could do it in 16:45).
Anyway since it sounds like you are just starting out with your overall fitness conditioning, I would suggest the following.. and keep it simple.
1 day a week do a long run. Since your goal distance in 3 miles, a long run for you would be 5-6 miles. When you do this run don't worry about your pace so much, instead just keep your pace consistent and your heart rate at 70-80% of your max.
Do 1 day a week of track work. The 6 x 400 is a great workout. The key to that workout is to run each interval (lap) at the same speed.. don't smoke your first two.. then half ass the rest.. and kick it up for the last one. For you starting out I would say shoot for 100 second per lap.. the first couple will feel too easy... but it will be hard to maintain. Take 100 second of recovery jogging between. Basically run a lap in 100 seconds.. then jog (don't walk or stop) for 100 seconds... half of a lap to keep it simple.. make yourself take the full 100 seconds even if it means doulbing back a bit.. then bam start another lap. Also 6 x 400 will be good your first week or two, but before the end of the month make sure that gets kicked up to 8 x400... a competative runner would be doing a minimum 3 miles of speed work.. but 2 is fine for you.. 1.5 is a little on the light side.
So there you go.. you have 2 days of set workouts.. You should only run 1 or 2 more days per week.. and both of those days should be active recovery type days... run for 45 minutes or so at a decent pace... faster then your long run day pace but far slower then your interval (track day).
That will build you a very strong base after 2-3 months. Very strong. When you want to mix up the Interval work just let me know and I can give you countless different track workouts.. all geared towards competing between 3.1 and 6.2 miles.
After a 3-4 year break I built back my running to competative levels.. it was rough at first, but a ton of fun. I've managed to compete in a half marathon, but still have not cracked the marathon.
I was always most competative at the shorter distances.. 400, 800, mile, mile relay... but I did do a fair share of 2 miles and of course a couple hundred cross country races...
Brian