Steve – first of all good luck and enjoy the mara. From what you have said, I would suggest adapting what you have now to your strategy on the day ie, use a run walk strategy. I use this consistently for ultras and for mara training and have mates who run <3:30 using it on the day.
Try running 9 mins/ walk 1 min in training. Approximates to a break each mile. This will ease the build up in mileage which you do not need to stress too much at this stage. Pointless injuring yourself now.
FWIW, my friends use (on the day) a 30 second (fast) walk break at every mile marker right from the start and this may drift up to 45secs to 1 min. They do this up to 20-22 miles, then just give it all for the remainder. They still burn out respectable times and feel relatively fresh.
When I did this in ultras, I was worried about losing time on others but it is interesting how you quickly catch up over the distance and feel much fresher from the short walk breaks.
Just a thought but maras are v personal things. Do what suits you on the day and good luck