Is your cardio all running or a mix? Have you built up to the time you plan or are you starting from scratch?
The fact you did a 5K on no training says a lot about your potential but you still paid for it.
The Couch to 5K
plan is the best there is.
Just jump in at whatever point you're at right now.
You should still be able to do the 5K you're planning but do a walk/run from the beginning.
Trail running is great but if the race is on asphalt/concrete, you need to do some training on those surfaces.
Have you been tracking your diet? I find that personally a sweet tooth means either not enough carbs to support my workouts or not enough protein to support recovery.