IMO I think treadmill is the best form of cardio. However, I also occassionally use the eliptical machine....when/if my knees are sore.
Here is some heart rate info you might find helpful(I did)!(the site is kind of cheesy but the info is good) Although, IMO its pretty hard to accurately find your heart rate using this method. But hey, practice makes perfect right!?
MEASURING YOUR HEART RATE
Wearing a heart rate monitor is an easy, accurate method of checking your heart rate... but you don't have a monitor. Here is another easy way.
The easiest place to feel your own heart beat is the carotid artery. Place your index finger on the side of your neck between the middle of your collar bone and your jaw line. (You may also use the radial artery on the under side of your wrist.) You can count the beats for a full 60 seconds or count for 6 seconds and add a zero at the end. If you felt your heart beat 14 times in 6 seconds the number would be 140 for a full 60 seconds. Counting for only six seconds is a convenient method, of course it is more accurate to count for the full 60 seconds. You can use several varieties of this method (30 seconds x 2, 15 seconds x 4, etc.). The longer you count the more accurate your reading. Whatever you choose, be consistent in your method.
TRAINING ZONES You can easily find your Target Heart Rate (thr) with this simple method. Subtract your age from 220 (226 for women) to calculate your Maximum Heart Rate (mhr). Find your
training zone below and multiply that number times your maximum rate.
Healthy Heart Zone (Warm up) --- 50 - 60% of maximum heart rate: The easiest zone and probably the best zone for people just starting a fitness program. It can also be used as a warm up for more serious walkers. This zone has been shown to help decrease body fat, blood pressure and cholesterol. It also decreases the risk of degenerative diseases and has a low risk of injury. 85% of calories burned in this zone are fats!
Fitness Zone (Fat Burning) --- 60 - 70% of maximum heart rate: This zone provides the same benefits as the healthy heart zone, but is more intense and burns more total calories. The percent of fat calories is still 85%.
Aerobic Zone (Endurance Training) --- 70 - 80% of maximum heart rate: The aerobic zone will improve your cardiovascular and respiratory system AND increase the size and strength of your heart. This is the preferred zone if you are training for an endurance event. More calories are burned with 50% from fat.
Anaerobic Zone (Performance Training) --- 80 - 90% of maximum heart rate: Benefits of this zone include an improved VO2 maximum (the highest amount of oxygen one can consume during exercise) and thus an improved cardiorespiratory system, and a higher lactate tolerance ability which means your endurance will improve and you'll be able to fight fatigue better. This is a high intensity zone burning more calories, 15 % from fat.
Red Line (Maximum Effort) --- 90 - 100% of maximum heart rate: Although this zone burns the highest number of calories, it is very intense. Most people can only stay in this zone for short periods. You should only train in this zone if you are in very good shape and have been cleared by a physician to do so.
http://www.thewalkingsite.com/thr.html