@DaveC: If you can't complete certain rides and are not enjoying it, why bust a gut, and put yourself at risk?
Yes, absolutely. This is the reason why I seldom try to keep up with the faster riders. On the other hand, I can complete a 200 now without much bother, sometimes even in a decent time. I enjoyed most of the Tayside Transgression, until the rain started and it got dark. I even managed a 300 DIY, but it was a fairly easy route.
I suppose it comes down to trying to push limits, that's where the challenge lies. Perhaps though I need to push myself to get a bit faster, before I go further. Which might mean I can eventually complete a 300 comfortably in time, instead of being up against the time limit.
@riffian: In terms of increasing my speed on the road, I've found club riding an effective and sociable way of doing it. I ride with PortoVelo who are a friendly club not too far from you.
I did actually ride with PortoVelo one time a couple of years ago. However I picked the wrong day: Edinburgh Marathon was on, and only a small group turned up, who were all pretty fast. No 'slower' group to tag onto as I understand is the normal practice when numbers are better. So I got dropped after about 20k!
I should probably try them again, if I can find time. In the meantime I need to do some training rides to practice hill climbs and sustaining speed...