For some they're an eyeballs-out race, for most I think they're a private test of their own ability, but for everyone they're a great chance to get out with like-minded souls, have a good crack, and invariably see some fantastic scenery.

This is my take on some I've ridden.  I hope it encourages others who might be a bit apprehensive to have a go.  If I can get round, anyone can!

105 miles - 2,900m ascent (approx).
The first running of the Cumberland Challenge was in 2006.
A figure of eight course on very quiet almost traffic-free roads around the northern Pennines, starting in Brampton (near Carlisle), and taking in the climb of Hartside, on to Alston, Garrigill, back through Brampton, alongside Hadrian's Wall and then briefly dipping into Scotland for a few miles before heading back to Brampton.

90 miles - 2,950m of ascent (approx).
The first running of the Bealach-na-Ba (Pass of the Cattle) Challenge was in 2006.
An absolutely magical course in the north of Scotland.  90 miles of quiet roads, great organisation, friendly locals and stunning scenery.
What more could you want?  Perhaps a 6 mile climb over the highest road in the UK?  They've got that too.

112 miles - 3,800m ascent (approx).
Regarded by many as the hardest challenge ride in the UK.
Starting and finishing in Coniston, this route passes through some of the finest scenery in the Lake District, mainly on quiet roads, taking in all the major passes (Kirkstone, Honister, Newlands, Whinlatter, Hardknott and Wrynose) - and some of the bits in between are tough as well!
If you fancy seeing what it feels like to climb Hardknott pass followed by Wrynose pass with 100 miles in your legs, then this one's for you.
Oh, and just to make it hard, we did this one on a tandem in 2006.