The furthest I've ever cycled so far is 147km from Rathfarnham-Dundrum-Saggart - Naas - Blessington - Sally Gap - Laragh - Roundwood - Enniskerry - Glencullen - Kilakee Road- Rathfarnham at 25km/h average speed, and honestly, at the end of that I was wrecked.
Would anyone have any tips for training specificaly for something like this?
Am I being too ambitous having cycled only over half the distance before?
Should I wait till next summer?
Here is the propable route I would take (my obvious lack of routing skills mean I've probably not got it right but it's vaguely there.)

http://www.mapmyride.com/routes/view...=1&new_route=1




