I think you are on the right track with interval sessions. For details, I'd suggest you check out the workouts and other information that vjohnson put in this thread.
http://trifuel.com/forum/25819/18mph-to-24mph-how-i-did-it

your run split is actually pretty good when comparing to your bike, so in that aspect you are using the right amount of power for your ability so you don't blow up too bad on the run. At the 7:30 time frame, the best thing you can do is just TITS (Time In The Saddle). Just by riding more (alot more) you can get that time down really low like sub-6 just by riding more.

The intervals will help too, but they won't be a darn bit of good if you have lots of power but can only hang on for 3 hours until you nose dive.

Bike endurance is a slow process, it takes years to build up the legs so that they can withstand 112 miles and still run fresh off the bike.

[quote=jarhead]Bike endurance is a slow process, it takes years to build up the legs so that they can withstand 112 miles and still run fresh off the bike.[/quote]

+1 Something's going on with a 7hr bike and 4hr run. It's might be not enough TITS, and (relatively) too much running. Think about your weekly training volume: 50% of time on the bike; 30% time on the run; 20% time in the water (give or take). Regardless, the bike should be getting the bulk of your time. As you suggested, I'd stick with interval training during the week, with long/slow bikes on the weekends.

To add to what's already been said (which is right on track), if you can do maybe one group ride a week with riders who are faster than you, that would help tremendously. I picked up a group ride about 2 months ago and already see an increase in speed and in endurance. I'm not sure I'd do more than that because you need the TITS and the longer rides, but this will help. You will also gain a lot of knowledge and tips on riding, etiquette and handling.