Howdy Simon, sounds fairly similar to my situation 3 years ago, I'd ran as a schoolboy, (though only running shorter distances) then managed to put on a load of weight, peaking at 15st7lb not long before turning 21... I just tried to get out and run here and there (2miles at a time was about all I could manage) and then chose a training plan to stick to - for that reason I'd suggest finding a reasonable training schedule that fits in with your lifestyle!
for a 10k, I don't think you'd really need to run too far on your long runs, though of course you'd benefit from longer runs if you plan to go on to race longer distances... just make sure you get yourself out the door regularly, and when you feel able make sure you throw some hillwork, speed work (intervals, tempo runs & fartlek) and any other strength building runs you can think of into the mix.
I'm not a fan of the runnersworld forum, but I can't deny they have a great library of training tips, and a large number of training schedules to choose from!
Hope you stick around and enjoy the forum
