Use an item to track you intensity, such as a heart rate monitor or power meter. Heart rate monitors are the best option because you can use the device for a multitude of exercises.
By using a heart rate monitor you can keep track of your intensity and progress over time. Keep an eye on your heart rate at the same given speed or, a particular route, during your fitness endeavors. Over time you will be able to track a change in heart rate versus speed. A lower heart rate at the same given speed is an indication that your fitness has increased.
Note: Try not to compare rides under conditions that may alter the data. Examples are: a tail wind will decrease you heart at the same speed and, just the opposite if you have a headwind. A really hot day will increase your heart rate versus a cool day.
Gear down. Use a gear that will allow you to maintain a pedal RPM above 80 rpm (preferably between 80-105 rpm). By switching to a higher rpm and lower gear you allow the heart and legs to work together better. When using a high gear and pushing really hard on the pedals it increases the muscular force required which fatigues the legs in a short amount of time. The high gear with low pedal rpm action also requires less aerobic effort but remember conditioning the heart is one of the main reasons we exercise. So, meet the two in the middle and ride in a slightly lower gear and pedal a bit faster. In time you will be riding farther and faster with more ease.
Proper bike fit. Correct biomechanical fit is important for your comfort, efficiency, and injury prevention on a bicycle. It is especially important if a person has past injuries/surgeries (i.e. neck, back, knee) that need to be considered. Wherever you purchased your bicycle from should have a qualified person to fit you and your bike. If you do have a special need that requires addressing such as injury/surgery, always consult with a physician before your fit. Your physician can tell you whether or not to consult with a qualified and trained professional that deals with the specific biomechanics of cycling. When a bike is comfortable a person is more likely to enjoy the ride. So, get fit and enjoy the ride.
Lastly, Always wear safety equipment and obey your local traffic laws.