Taking marathon running to the next level ( I hope!!)

There are loads of articles on this site and the web in general about training programs/regimes to build up speed based around interval running, tempo runs and on and on and on. I think you just have to adapt any advice to cater to yourself.
Personally, I really dislike the gym, dont like the air, the heat...I prefer just to get out and run, though there is benefit in building up muscles not normally heavily tested in running, but I believe you can still get all the major muscles sorted by mixing in different terrain, hills etc.
I also hate structured interval running, run 1K at X pace then jog at Y pace and so on. I run to warm up, speed up when I feel good, slow down when I dont then speed up again with second wind....almost interval running :) Though I know that interval running is supposed to be the 'way' to increase speed, it would knock all the enjoyment out of running for me if I went out to run like that, so I dont. Maybe if I hit a plateau and really feel the need I might try it, but it doesnt suit me. You are still improving so giving up cigs and sticking at it will probably still see improvements in your time.
For tempo running I enter races because its very hard to push yourself running on your own and you dont need to do too many tempo runs anayway. Main drawback here is expense.
I am operating on a marathon speed experiment by trying to build up my endurance. So I'm doing long runs, in the expectation that where I generally start to feel bad in a marathon aorund 20 miles should be extended to say 24 miles. This means that I'll be able to maintain a good speed throughout rather than fading too early or too much, but as I said at the start, thats whats working for me at the mo.