it's easy for your body to adapt to every day habits, that include riding a bike. So if you ride your bike 4-5 miles a day, after a couple months, it will feel like you only did a mile or two. The HR drops as you get fitter, even if you don't see the result. You can do this by getting a HRM. With the HR lower on the bike, you're not getting a work out unless you increase the intensity or increase the duration. Sometimes its good to do both. As reference; 5 miles at 10mph is 30min, 15mph is only 20minutes of exercise, or 20mph is 15mins.
Count calories, I like myfitnesspal. its free and has a good food data base. The calories burned on there is boarderline rubbish for me. I'd trust Strava/Garmin/Wahoo guesses better, but have to wear a HRM to get accurate number. As you start logging food, see what group your food lands into, fat, carbs, Protein. See what mix you are at already, and as you drop cals in, tweak protein and carb ratio until you don't feel tired.
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Rule #10 // It never gets easier, you just go faster.