Charley- I think the nearest thing to your needs is the ipod nano 6th gen, but not the brand new one that came out in the last couple of weeks.
The 6th gen is about 1" square and has a clip to fasten to the top of your shorts. It has a built in accelorometer which works as a pedometer (that's why its best at the top of your shorts).
The reason I specify the 6th gen model is that older versions need the footpod and the brand new model is larger and has no clip. There are plenty of 6th gen ones around and with the new one now being out, prices should be dropping.
The installed nike+ software can be used to record your run and you can either set a run time or distance (KM is better as it will let you know your time for each KM, but not if you select miles). Once you have finished your run you can download the data to the Nike+ website via itunes and get lots of information on the run.
This is not a SatNav it's a sophisticated pedometer and as such is not as accurate on distance covered as it depends upon a consistent stride length, which is unlikely on hills or trails. Ideally find a measured kilometre and once you have completed the distance you can calibrate the unit. When I compared the distance to a proper GPS I found that what it was recording as 5km was actually about 4.85km (Bit of a blow as this meant I was not as fast as I thought). However this was more accurate than trying to use mapmyrun.
To know the times and countdowns it gives you you would of course need to use it with headphones. It fades the music to give you your splits. If you install itunes on your computer there are stacks of podcasts that can be setup to automatically download every time a new one comes out and you can then sync them to your ipod. It is actually quite simple. The 8GB model is probably fine and you can get them for about £110. I would also factor in some sport headphones as the supplied ones fall out of your ears easily when running. The Ipod is not waterproof but on your shorts with a waterproof covering it, it should be fine for light rain.