Congratulations on continuing on your exercise journey. Coming back from a major surgery can be very difficult, but there are always ways to improve your health while keeping your modification needs in mind. Walking is a great way to keep your fitness in check and 30 minutes of walking daily (150 minutes per week) has been shown to decrease cardiovascular disease risk not to mention help you to feel better emotionally (ACE Personal Trainer Manual; 2010, page 4). Whenever you do exercise, it’s good to be sure that the intensity is such that it will elicit a positive change in your cardiovascular fitness. In whatever mode that you choose, try using the Talk Test to determine if you are working hard enough. Here is an article on the Talk Test, how to do it, and what it means: http://www.acefitness.org/certifiednewsarticle/888/.
Whether you choose to walk, bike, or do the elliptical, these three modes can all be used to improve cardiovascular conditioning. The fastest way to improve your VO2 max (or the amount of oxygen your body is able to utilized efficiently in a given amount of time and a measure of cardiovascular fitness) is by implementing intervals. Here is an additional article on how to program intervals into your workout routine: http://www.acefitness.org/acefit/fitness-programs-article/2929/ACEFit-workout-a
These intervals do not need to be all out sprints but should raise your heart rate a bit higher than your comfortable steady state pace. Begin with short 15 second bursts of slightly higher work rate then come back down to your steady state pace. As you progress in fitness, the intervals can become longer and ‘rest’ periods shorter. After you have increased the duration of the intervals, then begin increasing the intensity.
Keep in mind that you may also be able to use the preprogramed training tools already loaded into the cardiovascular equipment. This is a great way to keep yourself from getting bored with the same old cardio routine.