D2D- I'm a short, chubby 40-something year old, who only took up running in my late 30's- my marathon PB (only done 3) is 3:56-ish, but getting better- I would be wrecked if I did anything like your training. Maybe, as I build up fitness, I'll be able to increase, but it's got to be very slow, as I know from experience, that I just pick up niggles- mainly ITBS and foot problems- peroneal tendonitis, blisters++++, despite all the different runnig shoes options I've tried.