2 miles in under 20 minutes?!

Slow your pace down. 2 miles in 20 minutes isnt extrmelyhard, sprint the last quarter mile if you have to. Just go at a pace you can handle, then for the last quarter or half mile go your hardest

Run in your spare time.
Get home and run a mile.
Then get home the next night and run a mile and a quarter.
A mile and a half the next night.
Keep progressing gradually until 2 miles is normal. If you can do 1 mile in 7.30 there is absolutely no reason why you can't do 2 miles in 20 minutes. Especially if you are prepared to put time aside in your spare hours like I suggest. If you aren't prepared to do that, then there wasn't much point in asking for advice!
Also eat well (protein, fibre, iron), drink plenty of water and energy drinks, get plenty of sleep each night (especially the night before the assessment) and just do as much exercise as possible.
Common sense really.
The key point is that if you're genuinely struggling now, you won't magically be able to not struggle without a bit of hard work.

You obviously have to train. The more often you run, the more stamina you can build up and therefore the longer distances you will be able to run. Run everyday and eachday run a little further. Eventually you will be able to run two miles easily. Good luck!