Intensity

Intensity of effort during exercise is what stimulates your body for a fast response. If your exercise routine is not producing the desired results, one of the reasons may be you're not trying hard enough. Please do not confuse trying hard with putting the hours in. Sitting on a stationary bike for two hours 'spinning your wheels' is NOT going to get you fit.
If you did tabata style intervals on the bike (20 seconds full pelt, 10 seconds rest, 8 times. See video below) for a total of less than 4 minutes, I promise you that you would be fitter by the next workout.
You need to get out of your comfort zone to force your body to adapt. Your body is a product of evolution, it will only change if it has to. If your exercise routine is well within your capabilities, then why would your body respond, spending thousands of calories in the metabolic process to make you stronger, fitter & faster?
IT WON'T!
Push your body to its limit, with brief, intense exercise, then rest, recover & it will respond.
You can train long, or you can train hard, you can't do both. If you try, you will end up pacing yourself, which will lessen your results.
Next time you feel like going for an hour long jog to get fit, try the 100 burpee challenge.
It's simply 100 burpees (press up and jump) as fast as possible. You will see why your body must respond.

Let me know what you think.

John