Your rested heart rate in the 50's is perfect. You only need about one LSD session per week to maintain it now, and sport specific training is the best. You have that harder, longer term problem solved of building your aerobic base.
Now switch up to shorter, higher intensity cardio of your choice. Do a 4-6 week block and work progressively harder. What cardio best replicates down hill skiing? Do it. With consistent effort, you will adapt rapidly.
After that, maintain your gains with a session per week, and focus on weights again.
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