Andy,
as a T2 you don't have the same problems as I do, and I can only expand on my pre-diabetic experience.
It is perfectly natural to struggle at 2-2.5 hours, that's the "wall" where you've depleted your circulating and stored glycogen reserves adn you switch to the fat burning mechanism - it's more of a gradual decline in glucose and increase in fat burning, but after that time glucose is down to almost nothing. Fat reserves have a whole load of energy, but it's a slower pathwayand restricts intensity of activity.
My opinion as a well read amateur athlete is
1) carb loading is a load of ********, you can't store that much more that way
2) train your fat metabolism - find an aerobic heart rate and do your endurance training there - 180-age is a good starting point, use a heart rate monitor and stick to it. You will train your body to be more efficient, use fats and keep glycogen reserves slighltly longer, you'll also get faster at lower HR, so that when you need it you'll have a reserve to call on - less time spent in the red zone during training
3) after about 90 minutes start taking on regular small amounts of carbs (can also do it earlier if you want) to keep that circulating glucose up there. At 2.5hrs plus, you can't be comfortably ingesting the same amount of calories as you are using, so don't worry about weight unless you eat it all again afterwards.
Dave