One interesting question is how long does it take to determine that it's a bust? In running they said that a run doesn't suck until it has sucked for two miles. If you feel bad at that point, it's OK to pack it in.

I've had rides for a few miles when the legs felt like lead amd the lungs felt like a three week old kitchen sponge. Then all of a sudden my legs felt better and I got a good ride out of it.

Other rides have just sucked from start to finish.

Needless to say, yet I'll say it anyway, if you're blowing chow from the get-go, that's a good sign to head on back home.