Hi all
AM - I'm in the "run by feel" camp with Lm and LS, but I've dabbled with science in the past, including HR monitoring. I've never read anything that conclusively says that your MHR reduces with anything but age (btw don't place too much trust in '220 - age', its way off for me), whereas your resting (RHR) will reduce with fitness. However, what I noticed from average HR during races was that even during short intense runs like 5k and 10k - where I'd expect my HR to be pretty high throughout - this reduced slightly as I got fitter/faster, just a few beats and perhaps not consistent (every day is different). So what your seeing is probably an efficiency saving, you've probably taken a massive fitness leap with increased training and your HR is unlikely to get so near to your maxHR in any run. I didn't record maxHR very often during training, but I never usually got that close to it. If you feel ok during intense sessions, despite lower maxHR and the pace is comparable or better than before your marathon training, then I wouldn't be too concerned - marathon running is all about running efficiently and keeping the HR from drifting up as long as possible. When the marathon is done and you concentrate on training for something shorter and sharper you'll probably find that during the race you'll get closer to that MHR figure, but average will be lower than in previous races of the same distance, as your legs and core are more conditioned to being placed under strain.
LM - Hope it all went ok? Lots of twinges in a marathon training progarm usually, not many serious, though overdooing things is very possible if you increase mileage overall, or of your long run too quickly. This gets better with each marathon training campaign as most peoples average mileage increases so they retain better muscle memory.
LS - I'm expecting sub-8 for your next hill run 
CR - Good 10miling. With your fell running background, are you finding it hard to adjust to more running on tarmac/concrete? e.g. sore feet. After doing mostly trail for a long time, even a slight shift in momentum to harder surfaces has given me various aches and niggles.
bec - Don't panic. How much longer do you have to face the heat/humidity? Could you do some early morning or late evening runs? If you can face that and get some decent training in you'll find the marathon easier. Always suprised me how much the heat effected me at London marathon, even on not really hot days (15c or so), just because its a few degrees cooler in Hull usually, more breeze, the heat kicks up a notch in April and the heat generated by so many hot bodies around.
Since my last look in; SAT before last: 18.4m night/morning run on blubberhouses moor (nasty wind), MON: recovery run, TUE: 9m hill run, WED: recovery run, THU: 5 x 2k @ 10k intensity with 1min breaks and long Warm Up and Cool Down with club (very tough), SAT: 22m on canal towpath from Keighley to Leeds (flat, slightly downhill). Brings the four week average over 49m at the moment, best marathon training mileage I've ever done, easier paced and on the feet trail miles are really helping I think.