Specificity is key to improving on any fitness goal. For metcons, it becomes difficult because CrossFit is very unspecific, but probably the one thing you can work on specifically for this goal is time domain. Endurance is still a general term. Endurance in a 20 min workout is different than endurance in a 60 min workout. If you want to get better in the 12-20 min amraps, you need to spend more time in that time domain and learn how to pace for that time domain. If you approach the beginning of a 20 min workout the same as a 7 min workout, you will burn out. I think any endurance work in the 12-20 min zone will benefit your endurance in that time domain, but the cost to your body will be different.
Low impact exercises such as running, rowing, assault bike will allow you to build a strong base and be more recovered for the sessions with loaded movements. Vary between moderate and high intensities depending on what the programming has been like in your other workouts and how recovered you feel. Begin by focusing mostly on moderate intensity. A 20 min row @ 70% is going to be less taxing than ten 2 min intervals @85%, so you will be able to recover quicker and there is less likely hood of technique breakdown. Once form is mastered in the moderate intensity, add more high-intensity work.
Start out small. Add 1-2 days a week working in this time domain, slowly build up volume and intensity. The first several weeks and months should be focused on form before earning the right to add intensity. After building a solid base with low impact exercises in this time domain, you will be able to recover better from long duration workouts. That is when you start to add additional metcons in this time domain that include higher impact movement.
Also, play the long game. If you want to run a marathon and you haven’t run a mile in months, don’t sign up for a marathon in 12 weeks. Sure you may able to complete it, but you will just develop and reinforce bad mechanics over the 12 weeks and be crawling to the finish line. Same thing applies for 20 min amraps. Don’t start by doing Fight Gone Bad 3x/week when doing a single thruster correctly is near impossible.