What is your weather like? I ride in hot, humid weather with lots of hills, and a 90 minute ride is pretty taxing (I'm a 50 yr. old newbie).
Our weather in San Diego is very temperate. Today, for example, it's 65 F. My typical training ride is 1 hr. and 50 minutes, with 2,000 feet of climbing. If it were a lot hotter, then I'd probably eat an Extend Bar one hour into the ride.
During the ride, or after? If during, where do you come up with a protein shake?
Protein shake is after the ride, preferably within 30 minutes of stopping. This is supposed to help in keeping your body from using your lean muscle as fuel. Once you're not a newbie anymore, you won't find a 90 minute ride that taxing, especially as young as you are. I'm 63 :-) .