Good comments all, thanks!

I prefer to run alone, actually, and that gave me some insight. When I'm running, I think about running. Foot strike, pelvic rotation and leveling, femur alignment, arm swing; if I'm doing speedwork, then concentrating on keeping the pace (and wondering how much freakin' longer until the recovery interval ) 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.

Cycling, I don't want to think so much about my form. Partly because it's half decent already (or so I'm told), partly because the reason it's half decent is because of the race training back in the day, that I totally burned out on and do not want to burn out again. And I especially don't want to do speedwork or set fitness goals on the bike. I'm very wary of getting "too serious" about cycling just because of how hard I burned out the last time (didn't ride at all for 12 years).

Then, just because of the speed and balance involved, I have to be more aware of my surroundings on the bike - no rolling my eyes halfway up in my head as I can do in a running meditation. 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.

So bottom line is I think I said it myself in my OP, w/r/t commuting: I need a sense of purpose, I need a reason to ride. Living in the country as I do, "seeing nature" ain't the reason, either - I see lots more if I stay quiet at home. This year I do have a goal ride in the fall (Columbus Fall Challenge), so I guess I just have to focus on that and how disappointed in myself I'll be if I don't finish, and how much it'll hurt if I attempt it without being in shape. That's getting perilously close to "too serious." But maybe I just need to do it.

Keep the thoughts coming - thanks!