In reply to Wee Davie:
> 27/28 km/hr over 100km+ is a pretty big jump from my current standard. Kudos!
It's not so difficult if it isn't too hilly. I did a ride just short of 200 kms on my own a couple of summers back at 28.6 kmph average speed. Most of my rides in Finland I would be at or a bit below 30 average. But riding in England over the last month or so, on much more hilly roads than in Finland, and I've found that doing 25 kmph average even for just a shorter ride of 40 or 50 kms is really hard. The bombing down the hills at 60 or 70 doesn't balance out the time sweating up them in the granny gear doing 11 or 12 kmph!
So if you're doing 25 average over hills, well done!
Riding with others makes a huge difference. On a big sportive, I did 140 kms at over 32 kmph average and it felt pretty easy. If you get in a group that is going the speed you want, its just so much easier than riding alone and on a big sportive you might not even be need to go to the front of the group and lead for a bit.