i'm not the fittest level 4 ref but on several occasions i've passed the test following injury with 4 to 6 weeks of training. there's no training you can do now tom actually improve your fitness, infact i'd say do nothing so you dont tire yourself out
on the day my advice would be to run the first 2.5 laps hard, building up time in the bank fo when you'll be tired later. if you can, find someone to run with so you can stick with them, i always find it easier to run faster if i'm with someone to keep pace with.
aside from that, consider if you really need to run it this weekend, are there no other opportunities? there's nothing at all to be gained by trying and failing knowing you weren't fit