I've been doing a lot of research on this (and experimenting) as I'm getting into marathon distance mountain bike races with a goal of racing Leadville in 2023. Also, probably why I haven't been hanging out here as much, although I do have a MT trip I need to start thinking about.
I'd always been more of a lower carb person in my general life, but I've 100% realized fueling my workouts with carbs is leading to better performance, better improvement, better recovery, and I don't feel like stuffing my face for the rest of the day. If I do an easy Zone 2 / Endurance ride for 60-90 minutes I might do that fasted, but everything else now gets fueled. If upping your performance is what you're looking for, I'd start with a gel to start and then every 30 minutes.
Hydration is important too, I do roughly 1000mg sodium per bottle on the bike. If you're not carrying anything with you when you run, consider sodium intake before you leave too, it'll help you not just feel like you have to immediately piss out the water you just drank.
Most of you are going to think I'm crazy, but I'm taking in 120g of carbs (50/50 glucose/fructose) per hour on anything that isn't an easy ride (Running would have to be less - easier on a bike). You have to train your gut to take in carbs, and the body can only absorb 60g of glucose per hour so you have to be cognizant of what you're eating so it doesn't start causing you issues.