I'm less than half your weight and still get some issues. I'm currently dealing with Plantar Fasciitis but the combination of a strassburg sock at night, Scholl insoles and some painful rolling with a small frozen water bottle finally seem to have cleared it. I shall be testing it later in the week but generally I'd say stop and deal with any problem immediately. I naturally run on the front of my feet, I'm not sure if this has contributed to the problem I've had but I may try experimenting if it returns.
OP - good to see you running but do be careful. To be blunt 18st is a lot to be carrying around on a run, but good for you for wanting to. The key is to take it slowly, both speed and distance and back off v quickly if you feel any niggles. DO NOT RUN THROUGH THE PAIN. Pain is telling you something so LISTEN TO IT.
FWIW, I was running ultras and then had two bad years which saw me stop training and stick on 2 stone. Not good. Have just shifted one and half stone recently through exercise and diet. Was running 5-10miles of relatively hilly XC. Generally ok but go the odd knee twinges so stopped for at least a week and went riding, rowing or swimming instead. I still want to get down another half a stone before I up the miles again.
It really is important to avoid injuries when running. They put you back a long way and make you feel shit.
You can totally over think running IMO. The advice I have for someone at 18st is start slow, do the run/walk/run thing and preferably do it on grass or astro turf, where the impact will be lower. Also don't over stride and heal strike.
Way too much is written about trainers, just get some comfy ones that fit.
