This is the right answer.

Assuming you aren’t coming to your 2+ hour session at the end of a calorie deficit week, you’ll have enough on board energy for about a couple hours of decent performance run. Which I personally equate to being about the same as 2 hours bike + 1 hour run.

Personally I alternate long workouts (in particular the long run) so I am fuelling for one week (in order to practice and experiment) and not fuelling the other week (to train the body to efficiently convert stored energy for fuel).

If you ARE doing long workouts at the end of a calorie deficit week, then I’d certainly advocate you replenish your carb stored the day before and take fuel with you.