For most of us, the rule of thumb is "if you can't go long, go hard".
If you spend a total of 30-60 minutes each week in the "red zone", you should see improvement. Just don't do hard interval days back to back and make sure you get adequate sleep...you need time to recover from them.
BTW - they say it takes 3 years or so to reach your full potential as a cyclist, so you can probably look forward to substantial gains in performance over the coming season.