I've decided to sign up to two...
The first is the Winton 10k trail on 28 March ( wintoncastle.co.uk/activiti... ). Which is on a route that I do fairly often, being as it's in and around the village I live in, so I kind of have no excuse really (and it's in aid of our local primary school and playgroup.
The second is the following weekend, on 5 April, and is the 13,1k Edinburgh Kilomathon ( kilomathon.com/ ) which follows the network of cycle paths through the city and ends in Murrayfield stadium.
Knowing how muddy (and sometimes waterlogged) my local one can be, I'm hoping it's a reasonably dry second half of March. Just as well I have a pair of trail and a pair of road shoes I guess...