That's great, Catrin! My heart rate is always very high when running even on the easiest terrain, so running always feels a lot harder than cycling. I think you have good potential if you were able to run 20 minutes without your HR getting too high. Shows you can probably run a lot more as your body becomes more tolerant of it. When you first start to run, it takes time to get the tendons/ligaments etc. used to the motion, so it's best to increase time/mileage slowly anyway.