18stone isn't necessarily "seriously" overweight - some of us ex-rugby playing runners aren't far short of that but perhaps the difference is our muscle mass is greater than the norm so we are naturally heavier. I'm around 16.5 stone at the mo.
anyway, you say you don't have a faster or slower. although you think you have only one pace, in reality you don't. I suspect therefore that the runs you are doing aren't at the right pace.
if you're trying for a 5hr marathon, that gives a marathon pace of just under 11.5 mins/mile. have you tried to run at this pace? if you're finding that hard perhaps a 5hr marathon is out of the question; if it's easy then you're probably undertraining.
you need to be able to run slower than the predicted pace in the warm up and cool down sections, and many of your long runs will be at a slower pace.
weight might have nothing to do with this but generally speaking the lighter you are the quicker you'll go.
see what happens, and how it feels at an 11.5 min mile pace