I've bicycled 19 days in a row, and I'm starting to feel better and getting better at it.
I rode about 8 miles yesterday, while the weather was sprinkling and it was dark. Today I rode the same a little earlier so I had some light, and I don't feel tired, I feel energized.
I've been averaging about 12 mph for entire rides, including stops, which is supposed to be typical for casual bicyclists (as opposed to cyclists, competitors in the sport of cycling.) I don't keep track precisely of how fast or have a speedometer or bike computer, because that would defeat the purpose of getting out there and letting out energy freely and losing track of time. I blow past some more casual bicyclists despite myself. Since I'm up to using gear ratios of 2 or more through the whole ride including uphills and I'm increasing my cadence along with that, I might get fast enough to keep up with some amateur cyclists, just based on my natural level of enjoying physical exertion.
Exercise literally means "driving out" as in driving animals out of an enclosure to let them have their necessary physical activity. When I studied that and decided to believe it works that way - that you just get out there, then do as much as you like - then it became easy. I just make sure to get out on the bike at all every day, and the rest comes naturally.