Try
http://www.lvmv.de/content.php?link=LaufeninMV - official athletics association site with a list of races in M-V (click on Laufkalendar 2007 at the top). All in German, but I'm sure you can work out what's what! If you're in Rostock by the 13th May then the Rostocker Citylauf is a big, mass-participation race with a few distances (3km - half marathon) and a few thousand entrants (see
http://www.rostocker-citylauf.de/)As for trails - my recommendation would be to get yourself the
Kompass map of the area which shows all marked walking trails (getting a train out West and then around Bad Doberan, in the Kühlung or in Hütter Wohld is nice, or get out to the Rostocker Heide if you're afraid of hills). Or of course just head out to Warnemünde and run along the beach/coast for as far as you like! No worries about disturbing people, or at least I've never had trouble. If you're wanting to run off-tarmac in the city then there's a few smallish parks, or along the bank of the Warnow opposite the Stadthafen are about your only options. Otherwise there's plenty of cycle-ways and paths around.