Dave, you should to find a short adventure race to do!
I did my first one last summer at it was fantastic! All said and done it was about 90km between 5-8 hours, part cycling, running, hiking, canoeing and some obstacles. You see very fit and if you know how to ride a bike you shouldn't have to train to much, i spent most of my time training to use a compass and map properly haha
Not sure if there are any in your area, but they are well worth the search!
Thanks, that's a good idea, I fancy something with a bit more interest than just following a signposted route, a bit of navigational challenge as well.