It seems like you are already in good shape. 40 mins of cardio 6 days a week is a lot. Do you find the elliptical really easy compared to the treadmill?
If you start off really slow, say at a 10 minute mile, which is a fast jog or a really slow run, going out 4 times a week for 40 minutes a time is 16 miles a week which is pretty good. If you used to do 25 miles a week though I think that you can handle something above a beginners program.
The main thing with the intermediate runner is the long run. 2-3 short runs during the week and then a long run, usually double, on the weekend. You can skip a short run during the week but not the long run. With your history and the fitness level that you are at right now you would be ready for a 10K by week 7 or 8 and if you really do run 4-6 per week and you can do 40 minutes of cardio without stopping right now, you could do a half marathon by the end of the summer no problem.
This is what I suggest to start with:
Mon: 3 mi (at least 30 mins so if you run faster you might do 3.5mi)
Tues: 3 mi
Thurs: cross train
Do this for 3 weeks and if/when you can do the 6mi without getting exhausted or you can do it in under an hour move up to:
Thurs: cross train
If you hit this mark by week 4-6 then you are ready for a 10k.
Your outdoor track is 400m so 5mi is 19 times around the track. You will get bored pretty quick so scout out some nice roads. They should be flat. Once you hit the 1o mi weekend runs you can start working in hill and interval training.
The other thing is that you are a big guy and this is a lot of running so I would suggest some good shoes. Good stabilizing shoes are important for bigger guys. A stabilizing or motion control shoe keeps your foot in the right position which is important for big guys. You should probably go to the running room and have them examine your stride to see what kind of shoe is right for you. It's not essential that you do that right away but when you start hitting the longer distances the wrong shoe can lead to injuries.
I hope that helps and I hope that you keep me updated on your progress