My personal challenge for 2011/12: travel the entire length of the River Wye under my own steam.  From Plynlimon to Chepstow, it's about 218km (136 miles) long.

The Wye Valley Walk conveniently provides a nice waymarked route to follow.  I've got quite a pile of route guides, maps and leaflets, and a 'passport' to get ticked off at various checkpoints (mostly pubs, hah) along the way to get a certificate at the end

There's not really a fixed schedule for all this.  I'm not doing it 'in order', I'm not doing it all in one go..  and I'm not going to walk the whole way!

There's some good whitewater bits for kayaking (as well as flatter sections we often paddle in the summer with beginners!), and NCN 8 follows the river quite a bit between Hay and Rhayader, giving good opportunity for biking. 

It also turns out that footpaths and cycle route increase the total distance to more like 235-240km.  Which just makes it more of a challenge

I'm mostly doing this by myself, so walks will probably mostly need to be circular (so slower progress), or make use of public transport where available.