You would need much more than just long jogging sessions to go to 20km, like tempo runs, strength, power etc. My advice is to do:
1. Couch to 5K
2. Beginner 5K https://www.halhigdon.com/training-programs/5k-training/novi...
3. Beginner 10K https://www.halhigdon.com/training-programs/10k-training/nov...
4. Intermediate 10K https://www.halhigdon.com/training-programs/10k-training/int...
5. Intermediate 21K https://www.halhigdon.com/training-programs/half-marathon-tr...