For most of us, the rule of thumb is "if you can't go long, go hard".

Practically, this means I always try to get in some intervals early in the week (assuming I'm adequately recovered from weekend rides/races).

Based on my research, and personal preference, I've found it best to focus on semi-structured 30 second and 4 minute intervals.

For the 30 second intervals, I go all out - they should hurt and you should be spent at the end of the 30 seconds. I typically allow for a one minute recovery, and then jump again for another 30 seconds. Start with 4 of these and work up to 8 or 10 (or more).

For the 4 minute intervals, try to find a pace that's about what you could hold for 20 minutes or so. They should feel pretty hard, but not quite all out. Allow about 2 minutes between intervals and then go again.

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.