Suffering and aching is fine... but pain... especially sharp stinging pain is not a good thing. For optimum fitness improvement, you have to incorporate a lot of different workouts, not on the same day:
REST: easy day off completely, or a really easy ride
ENDURANCE: long steady-state ride to develop energy-delivery, fat-burning
TEMPO: aerobic workout close to LT to develop heart & lungs
INTERVALS: on flats, hills, short/long, timed-sets or pyramids to develop muscular-strength & efficiency
SPRINTS: all out 100% efforts, no holding back.
There are only a few times when you're gonna be "hurting". If you are in pain every single ride, that's way, way too much and you'll end up overtrained and fatigued. A lot of people get stuck in the middle of no-mans-land in training where they're not getting enough rest and not doing enough miles, and at the same time, they're not going hard enough. So a lot of times, you have to slow down and do real endurance rides (3-4 hours) and on another day of the week, do sprints and intervals and keep the mileage really short, like 1-1.5 hours max. There's no one "right" answer as far as pain goes...
Last edited by DannoXYZ; 09-04-06 at 08:20 PM.