20 minutes is what it takes to get your body to the aerobic phase.
ANY amount of exercise is better than no exercise.
The additional minutes you tack on at the end will burn calories but will not really add the aerobic aspect, which is fine since you already did the first 20 minutes initially.
Exercise should be fun (to a degree) so changing things up is great. You mentioned your feet cannot do hard impact....does that mean walking is out? Walking is about the best thing you can do and enjoy nature at the same time.
Since you have just started, give yourself some time. When your body feels better and better, you will enjoy doing more.
Have fun and congrats!
Thoreau said, 'A man is rich in proportion to the things he can leave alone.'