I would think you should spend most of your time simply riding at below your ventilation threshold, putting away a lot of mileage for three months. I.e. easy riding, starting with distances you can manage at the moment, and slowly increasing them. When you've done that, then you can start to think about the higher intensity involved with club rides, intervals, etc.
The core strength you need will still be built by just going out and cycling, but not as quickly. To avoid injury, most of all you need to make small increments in workload at a time, never large jumps in intensity or duration.
If you're really keen on getting better, then get a coach. Just ask Gonzo how effective a coach is.