You will find when you read up that there are different training zones for different purposes.How does your body feel when you're exercising just hard enough to continue to improve? I've been riding about 13 miles every other day and trying to pedal just hard enough to feel a continuous mild burning in my quadriceps. I don't push it past that. I haven't got too out of breath doing that - I think my muscular fitness needs to catch up with my cardiovascular fitness, maybe.
Right now you are probably at a stage where your muscular endurance is not very high yet - there will come a time when you can avoid burning muscles (i.e. lactate production) for a whole long ride....
You have the long moderate rides which mainly affect your metabolism - you train your fat burning system.
And then you have the strenuous work - hills or intervals - that build muscular strength.
Both of these variants make you improve, but they feel very different.
People usually go by heart rate but if you don't want to buy a HRM, never mind.
Just have a few different routes of different lengths and intensities and eventually you'll know your body and how it "feels" at different levels of exercise.
And you'll know you have improved when you can ride 26, 39 miles... etc... etc. It never ends...