Do you jog just yourself often? Or some other regular sport activity that you do alone? If yes, maybe that might help abit.
For starters, if I bike at night/when dark (which could be early morning), it truly is because I have a specific purpose in mind...to get to my destination. And usually whenever I've cycled at night, the distance tends to be under 20 kms.
So all the solo riding is primarily during daylight. Since I've been cycling regularily since 1992, I've gone through different phases of motivation. But there are several compelling things that still keep me cycling regularily solo about 40% of the time (since my partner isn't always around and any cycling friends I had, they have other personal schedules which made cycling together infrequent.):
*I have a few stock bike routes that I choose from, depending on weather and time of year. And my motivation. I do tend to prefer routes that have at least 1-2 hills, long quieter road segments and with something interesting/lovely to see along the ride. Even if it's the same old scenery..it changes according to time of year, weather, etc. I feel very lucky where we live now, there are some beautiful regular bike routes that I cobble together.
*allow my body to feel the cycling sensation every day. So that I get my body hooked on frequent cycling to a point, that body yearns to cycle on a lousy weather day when I haven't cycled. So after non-cycling day(s), I'm more motivated to return cycling.
*car-free life. Man, when a person is forced to buy groceries on bike, do other errands, it does keep me on the bike solo more often. I definitely cannot let myself off the hook to get the stuff that I want/need to use.
These past few months I've been cycling daily approx. 32-42 kms. per ride. Over the years, have done few solo rides that were 70-90 kms. each. Just routes I made up. For longer distances, I do tend to stay longer on the bike if I ride with at least 1 other person.
I'm not sure what else new I can offer to comments others have made already. I've always tended to be slightly hermit-like, a friendly one.I was the nerd kid who loved spending hrs. by herself on some art project or cycling in the cemetary (because it was a safe place that was relatively car-free to ride).



I was the nerd kid who loved spending hrs. by herself on some art project or cycling in the cemetary (because it was a safe place that was relatively car-free to ride).
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) etc., etc., etc. Sure, on a longer run my thoughts will drift, and if I'm on the beach where I don't have to pay much attention to my surroundings I may do specific meditations, but mostly I'm thinking about running.
Yeah, in that sense it is like motorcycling, but I don't feel like I need to be hyper-aware 100% of the time on the bici as I do on the moto. (Maybe I should.
) But the difference that I think is most relevant here, between the bici and the moto, is that on the moto, when something hurts I know I'm doing it wrong.
If all I had to do all day on the bici was scream downhill at 40+ mph I don't think I'd have any trouble with motivation. But there's the little matter of getting up those hills. 

