It's difficult to put on a definite label over the internet, Walter, but top of my list is a condition called trochanteric bursitis. When you get a lot of repetitive traction on the tendons that attach the muscles of your thigh to the greater trochanter (the sticking-out bit at the top of your thigh laterally), or the short tough ligaments that hold your hip in its socket, you get inflammation and pain develops.
It's a soft tissue rather than a joint problem, and anti-inflammatories (though don't take pills to allow you to train - listen to the pain), ice, and cutting your mileage may settle it down. But it would be worth getting your GP (or a sports physio) to give your hip a good going-over. If it is what I think it is, a steroid injection can provide excellent pain relief and get you training again.
As overuse is likely to be the cause of the problem, it would be worth reviewing your training schedule, reducing your weekly mileage (you should be starting to taper now anyway) and substituting a non-impact activity like swimming or rowing for some of your sessions.
Good luck with the New Forest Marathon. As long as your symptoms don't get worse between now and then, my advice would be to take a painkiller on the day if you need it and run, not to pull out.
Cheers, V-rap.