Hello mountainsloth...

I'm far from a qualified expert, but my two-penneth as follows...

Firstly it depends how much actual time/days you have in which to train.

Secondly, my strong opinion is not to completely throw out all of the gym work. I'm lucky/sad/stupid enough to train pretty much every day, and at the moment only 3 of those days will be solely dedicated to running, either club nights or long weekend run. The other 4 days are gym sessions (which will include a treadmill session or cross-training) incorporating strength work and core work. The 'runners physique' will come from diet as much as type of training (if anything diet is probably a bigger factor in your weight than training is).

But yes, increased mileage will help. But don't complicate it too much. You will be surprised how far an increase in just steady running can get you. Once you've got a good base (whatever your interpretation of that may be), then can look at putting faster sessions in.

Finally, for your ultimate goal of an AM/AL, you will need to get used to the constant barrage of ascending and descending. Regular racing and training in the hills are surely the best option. I do a lot of steep treadmill work due to living far from the fells.

I await someone with the actual real knowledge to blow my opinions out of the water. But my thoughts above are based on a vaguely similar situation to yourself (I was relatively new to fellrunning, from a background in fellwalking and competing at a good level in a different sport, and was inspired to run the AL races). That was two years ago when I started, and I'm now running about 15 races a year, 75% of those being AL's or long-distance type fell events (with varying degrees of success and failure).

Pete