Muzza,
I've run Berlin this year and last. We stayed in Wilmersdorf in a Pension Bella on Pariser Strasse (the nearest U-Bahn is Spichernstrasse I think) and prior to that also on the Pariser strasse Pension Elton. Of the two I preferred the latter, both did pre marathon early morning breakfasts, and its about a 10min walk to the start in Ernst Rohm Platz.
Around Pariser strasse there were a number of really nice little restaurants, and bars yum. The marathon itself is well organised but get to the expo early on the Saturday before it becomes packed, and don't forget the free pasta. You get given a goody bag before the marathon, and this year it had some useful stuff in it. A really nice fabric adidas bag (to shove your dirty running clothes in) and a wrist band to store keys in.
After you finish you can get your results printed out immediatly and you get free beer!
There isn't a half run at the same time (well there wasn't this year!), but there is inline skating (starting earlier than the runners). Beware there is no klaxon/gun at the start they just play music.