[pandaman]

>With small/insufficient muscle size, you simply run out of stored glycogen before you get tired cardiovascularly.

You can eat carbs during cardio ("fueling") though it's unlikely an issue doing 600 mins a week. Muscles store 15g of glycogen per 1kg (more for trained athletes) , which amounts to 60 (k)calories. In aerobic process with COP ~ 25% these nicely convert to output energy of 60 kJ. To produce this much output over 90 minutes you need to push power of ~11 watt. Elite athletes have FTP (functional threshold power) around 6 w/kg. It's over the entire body mass, not just muscle, but even if you are pushing 50% body fat, you can be pretty confident you have enough glycogen for 90 min of aerobic exercise at your FTP (also liver holds/can produce on demand quite a lot of glycogen and aerobic process will use fat for energy as well). Even if you do 200 minutes 3 times a week instead of doing 90 minutes every day you can get by staying in under 3 w/kg power zone, which is still greater than most people's FTP (and even fewer people can hold this power over 3 hours).