Firstly, don't worry about lining v reffing, you have at least 5 years to even worry about the pathway and you will never get that far if you're not a good referee.
All of the modern advice for fitness is high impact intensity training (HIIT), or interval training as it's also known. There are lots of articles all of the internet including football specific ones that run box to box, corner to corner etc. The secret is to knacker yourself out in 30 minutes, push yourself over time and you'll get fitter.
As a guide, I think the FL Assistant tests have a benchmark of 40m sprint in under 5secs (standing start).