Well having once weighed 16st 10 and now 11st 0 (actually, last weigh-in was 11st 1, but that was after having five hot-cross buns for breakfast, more than my usual bowl of chocolate porridge
) - I used to think I was "too big" for running, and certainly before I knew anything about running at a sensible pace and building it up, I would get into the habit of trying to run 3x/week (too fast) and ending up with shin-splints. So my approach was to get down the gym and partake in activities which I knew didn't leave me niggly or injured, namely cycling/spinning, other cardio, and subsequently weight training/core exercises. One obvious thing here though - if you decide to add any kind of exercise time to your weekly schedule, do it gradually, or substitute for other things, e.g. substitute a gym class for a run till you get a bit fitter, then add the run back in, etc.
I started running "properly" once the first three stone had gone and it just felt a little more comfortable, BUT if I'd known at the outset all that I know now about running, I probably would've given it a go much sooner, and just built it up more sensibly. IMO it's worth mixing up your exercise with different activities for a number of reasons. Firstly, you're not putting too much stress on any one thing (e.g. your knees, hips or running muscles). Secondly, your overall goal should be not just weight-loss but overall fitness, including improved aerobic fitness and strength - go for a run, then the next time go for a swim. Go to the gym a couple of times a week and lift some weights, or book a pilates class for core strength. The larger the proportion of muscles you're stimulating, the more you stimulate metabolism and calorie burn. Finally, if you're in the habit of doing different things then if you have any niggles from running, you'll find it easier to cross-train with another activity to keep the momentum going.
Nessie's right about the nutrition side of things as well because the two sides of the calorie equation are inextricably linked. I've never joined an organisation but did spend quite some time educating myself about nutrition, i.e. not just nutritional composition of different foods but the best time to eat them in relation to your exercise activity. Eat to compete.
So I suppose my short answer (i.e. opinion based on personal experience!) would be - keep running as long as it's not causing any problems, but mix it up with other activities as well.