Ok, I'm gonna ask some dumb questions....

1. What makes a road feel dead? Got a section on the M2 that consistenly feels like crap.

2. I cant find the balance/co-relation between HR, speed, cadence and pain.
What i'm tryin to say is, sometimes...(on the same ride)
My HR is dropping into 150's but my pain level is high indicating i'm trying hard, but should I push harder to work respiratory system and get HR back up?
My HR is staying high (160/170s) but my legs feel strong, indicating I'm trying hard, but should I push harder to work legs?

The reason I ask is I intend to start training using intervals and trying to work it all out.
Sometimes it feels easy to sustain 170bpm and have strong legs on an undulation, other times it feels hard to sustain 155+ and speed and cadence (whole rythm) gets shot but legs are on fire!
Am I overthinking all this crap and should just ride my bike and smile?