As always, your mileage will vary, but here's my current long run routine, training for a 50 miler.
1) Try to start run around 180 bg. Ideally, I do this by waking up at 100, eating a Clif Bar, and going out about 30 minutes later without taking insulin.
2) Hourly I test and eat another 40g of carbs, usually a Clif Bar. I also carry a flask of HammerGel and will take some of that if I need more sugar or want something besides the Clif Bar.
3) After 2 hours, I switch to Clif Builder Bars, to get the protein. Regular Clif Bars have much less protein than the builder bars.
I'm on the pump and keep my basal rates set at regular rates... this allows me to consume more carbs.
While there are many reasons for stomach problems, protein is often harder on the belly. That's why many strategies will have you only add it beyond 2 hours. For the first 2 hours, you can get by on mostly carbs.
I'm also a fan of Hammer Products. A strategy using that would be to take HEED (their drink) for the first 2 hours, and switch to Perpeteum (which has protein) beyond that. I've done marathons on only Perpeteum, but if you have stomach issues, that might not be wise.
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Great advice. I've been having difficulty figuring out where to set my basal on long runs. Keeping it the same and upping the carbs makes sense. I plan to run another marathon this year and will try your approach on training runs.
Good luck Potey - let us know how you do.