You may find it better to exercise using an interval program with regular rest periods rather than trying to exercise continuously without any rests.
For example, an interval program might be: walk for one to two minutes, rest for one minute and then walk again. You may need to repeat this interval many times to achieve at least 20 minutes of total walking time. Don’t worry if you cannot achieve a total of 20 minutes the first time you exercise. You can aim to build up to this over time.
Interval exercise programs have many advantages. These include helping you to tolerate your exercise routine better as well as enabling you to exercise at a higher intensity, which can give you a greater improvement in your fitness.