Your stats are very similar to mine, I bounce along at around sixteen stone..ish, RHR is low 40's, but before injury, running was my main focus, you will lose weight, providing you are nutritionally aware, I work on 80% good and 20% bad food intake, shift work doesn't help.
Your 3.30 time is very doable, maybe even quicker. If you want a fast time, choose your course wisely, popular and congested races, despite chip timing, will add minutes to your time. Do you prefer, flat or hilly? Fastest or one of the fastest is the Fort William marathon near Ben Nevis, OK, get up off the floor, the race is pancake flat, limited entry, and cool, all of which make for a fast time.
For a 3.30, you should be looking to tap out, 7.30 to 7.45m miles, towards the end it will trail off, aim to run as close to a negative split for the two halfs.
Training wise, depends on time, and focus, and how you want to arrange your family life. When I was keen, I would have one long run a week, between 13 to 16 miles, this was essential, any further than 16 miles your body will take far more time to recover, About a month before the race I would do a long run, maybe 22 miles, then a week to recover from that, two weeks at moderate pace/distance and a week taper.
Most of my runs are done at lactate threshold (que much debate, on what it is, blah blah), and I would mix this with the odd race, 5/10k road,fell, duathlon etc, just for speed.
staying injury free is vital, if you get pain,twinge, etc, go through the basics, RICE, etc, better to stop and check than bugger yourself up.
Look, at a series of races that you can guide yourself towards the marathon at, and try to run them at just above the speed you need for the marathon, if you need to do 5min per kilometre for the marathon, then your 5k should be sub 25, 10k sub 50 etc, if you can't match this then rethink your strategy
keep a log, be as anal as possible, hr, weather, pace, food, if something alters check the log the answer may be there.
It does bug me when people give advice, without proving their credentials, so that you can decide whether these are able to back up the advice, or if I am talking rubbish, it allows a more informed oppinion. My fastest marathon was Leeds, a rather hilly two lap affair, sub 3.15, splits of 1.36 and 1.38, take away the hilly factor and it could have got close to 3hrs. So yes a sub 3.30 for you is definately on the cards.
If you can, do some fell races over the winter, brilliant low key affairs, good laugh, usually finish at a pub, and fantastic training. Best of luck, choose a marathon that you want to do, I thoroughly recommend the one at the end of Ironman Austria.
