As most non-cave dwellers will be aware, this weekend sees the tenth running of the legendary Mountain Mayhem 24 hour race. But big though it is, not everybody will be doing it. If you're looking for something a bit different this weekend (and you're both very fit and slightly bonkers), then maybe the Pennine Duathlon may fit the bill.

The Duathlon runs alongside the Pennine Challenge, a 100 mile trail running race. As the name suggests, though, there's more to the Duathlon than running. The format involves a 26.2 mile trail run (that's a full marathon distance, off-road), followed by 100 miles of mountain biking. And then another 26.2 mile trail run to finish. On the way, the course racks up a monumental 13,000m of climbing and descending.

As if that doesn't sound silly enough, you'll have to be entirely self-sufficient and deal with your own navigation, much of it at night. Oh, and there's a 36-hour time limit. Need more gauntlets? Last year, just two people entered and neither of them finished. Take part in this one and you're definitely at the cutting edge of endurance lunacy.

All of which brings us to occasional member of the ever-rotating collective that is Team Bikemagic, Carl Silver. He's spent months training for another slightly ludicrous event, the Three Peaks Yacht Race (in a nutshell: sail as close as you can get to Snowdon, Scafell Pike and Ben Nevis, cycle to the foot of each and run to the top) but his team suddenly lost the use of the yacht, which is a bit fundamental. Not wishing to waste the training effort (and having already committed to raising some cash for charity), he's entered for the Pennine Duathlon.

Carl says, "Hope House was our chosen charity for the Three Peaks Yacht Race. We visited them during the preparation for the race and were very impressed with the work that they do, looking after terminally ill children and helping their families. Although they provide fantastic value for money, they still need to find a substantial amount of money every year to keep running. A very worthy cause..."

More details on the Pennine Duathlon (and entry forms, if you fancy a last-minute go) at www.ambitionevents.com/penninechallenge/. And you can send some digital spare coppers charity's way at www.justgiving.com/carlsilver.