which one are you thinking about running? London?

personally speaking, i wouldnt go straight for a marathon, i would start low and work upwards in distance

there are lots of runs you can compete it, such as race for life for charity etc, then look at increasing distance such as half marathons.

give yourself plenty of time to train and build up resitance and endurance

you also need to eat the correct foods, and try and find someone of similar fitness/ age to run with, it makes it so much easier.

Edit: as your in a big place, i would potentially try and find a running club with people of all abilitys.
plus there will be lots of events in brum to run in