Something about bicycling

So, I was riding on the trainer tonight, watching a movie...incidentally not a very good one...and I remembered something about bicycling.  Riding a bike on the road, for me, is like surmounting a series of mental obstacles:  I usually hit one about 10 or 12 minutes in, really when I'm just getting warmed up, and I feel like something must be wrong, I can't really be tired already.  But if I were to judge from my senses, I'm definitely not going to be able to keep the pace.  Usually I'm riding with a bunch of other people, so I'm ashamed to slack off, and I keep going.  At about 16 or 17 minutes the feeling disappears and I feel fine.  Making the mileage at a decent pace on a long ride is just about coming up on a series of these kind of physical/mental hurdles, realizing they're primarily mental, and continuing to be disciplined about keeping up my cadence, judicious gear selection, timing and keeping the pedals moving.  I definitely have physical limitations due to my lack of fitness (such as the two muscle cramps you can get while moving 17-18 mph and tethered to the pedals-good times)...but the obstacles I'm talking about aren't what limit my mileage, they just make me think its time to quit.