It’s vitally important to fuel on the long runs. Yes, you can run 2 hours without fuel, but it makes the long runs more difficult.
It’s surprising how long you can do a zero basal when you are running! I have gone over 4 hours without basal, only taking boluses for fuel. And the boluses for fuel are small, like my I:C can be crazy, like 1:300 or 1:500 or even more. (Sometimes I can fuel without a bolus, even after zero basal for several hours.)
Tandem has a quick-bolus button that can be used. This would be a very simple trick!
The Tandem quick-bolus can be set to either units of insulin or grams of carbs. Before your run, you would change it to carbs.
When you have it set for carbs, the Tandem quick-bolus increment options are 2g, 5g, 10g, or 15g of carbs. I would suggest starting with 2g. Just as a starting point.
You can change your I:C and make it whatever you want if you need to. But you might not even need to change it.
Suppose your I:C is set to 1:20. If you have that I:C setting, and you have a 2g quick-bolus setting on your Tandem. When you hit the quick-bolus button, you would get a 0.10 unit bolus.
This is important - you are not actually taking only 2 grams of carbs. You would take something more like 20 grams of carbs! But you are only telling the pump it’s 2 grams, so you get a super tiny bolus.
After an hour of running - and while you are still running - what does 0.10 units and 20 grams of rapid carbs do for you? Let’s find out!
(I can tell you that those numbers would not cause me any problems. I might need to take a little more carbs, maybe less. But those numbers would not crush me. And I am speaking of rapid carbs here, like a running gel, not a granola bar.)
But regardless, you start with something in that ballpark and see if you need to adjust it after a few trials.
Since you can’t make your quick-bolus option less than 2 grams, if you need less insulin you could just adjust your I:C on the pump. Essentially, the combination of quick-bolus carb options and your ability to change your I:C makes it possible to use the quick-bolus button for whatever you want, very easily and quickly.
So the summary is that you would have zero basal going for your entire long run, and try to fuel during the run with very small boluses and carbs.
Does all of that make sense?
(Yes, you can do it without the quick-bolus button, just using the normal pump interface. But I think touch-screens suck when running! Why not use the quick-bolus feature?)
(Some of this whole idea came to me when I was exploring other pump options after Omnipod abandoned their old PDM (push-button) and went to only touch-screen options. I knew that would not work for me, so I looked into Tandem as an option. I got a little insight from @elver about it too!)