Depending on your training history your legs may be feeling that for a day or so. 20 steps ain’t very much but you gotta use what you have.
The best basic aerobic (to improve AeT) stimulus is to keep the intensity moderate for an extended period of time. You need to fatigue that ST fiber pool. Fatigue of ST fibers happens mainly by depleting the intra muscular glycogen stores. Since those lower power ST fibers have great endurance it takes a long time to do this. If you go too fast then the faster twitch/higher power fibers, that you need to go faster, become fatigued earlier (cuz they have lower endurance) but that vast pool of ST fibers, the ones that act as the aerobic base don’t get the kind of training stimulus they need.
But it sounds like you did that and your legs will let you know,