Without wanting to repeat what's been said, I think that most people find motivation to be a fickle friend, whereas habit is your homeboy
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To train consistently you have to schedule it specifically, rather than have vague notions of "i'll go for a run today" so that you dont waste time having tiresome internal arguments and just get our there and do it, whether you particularly want to or not. This is important because to have runs that are awesome, you need to battle through the runs that suck.
it's like work- unless you're a shameless skiver on your last warning, you could probably wake up any morning, call in sick and get away with it, but it doesnt occur to you to do that as going to work is just what you do.
I do a mixture of lone runs, running with friends, and a club track session. I prefer mornings, so if I need to do an evening run I'll try and find some company so that I have to show up.
Weirdly, I dont find races that motivating (totally personal) because 1. you can just DNS, 2. you can get into a cycle of denial and fear where you dont want to train because you dont want confirmation that you havent done enough training. 3. If you race a lot it can be hard to get into an effective training routine due to taper and recovery cycles.