There's always faster riders, so you're always "slow" compared to someone.

Different people start out with different fitness. They may have trained at another sport, or they may just be genetically gifted. It doesn't matter, you just do what you can.

As a new rider you'll do best by getting in more time on the bike. Do more rides and do longer rides. Rule of thumb is to not add more than 10% a week. I would not recommend intervals at this point. Even most dedicated racers are not doing intervals right now as it's the end of racing season. But as a beginner you lack endurance, and you get that by putting in more time. Once you have that endurance base, and need them for training, then you can do intervals.

Also, different riders are better at different kinds of terrain. Very generally, larger riders are better on level ground while lighter riders are better up hills. It could be that your build is less suited for the ride you were doing than your friend's is. Or she could be better at drafting than you are. That just takes practice.