Improving Upper Body Endurance and Comfort on the Bike
We don't need much upper body strength - just enough to hold us up on the bike! Doing 12 - 20 reps of one pushing exercise and one pulling exercise is plenty. Since you're building endurance, do these exercises most days of the week.

Push-ups: Add a second set of push-ups. Start with hands 3 - 4 inches wider than shoulders. Lower until chest is just touching floor and then push back up. Keep back straight through full range of motion.

Single arm rowing: Increase the resistance for single arm rowing. Put your left knee and left hand on the bench, right foot is on the floor. Back is parallel to the bench with head up, looking forward. Right arm is hanging toward floor, holding 15-25 lbs. I use a bag of tin cans. Raise the weight straight up to your shoulder/chest joint. Keep your back and shoulder motionless; just use the muscles on your right upper back. This is a particularly important exercise for strengthening the shoulder and neck muscles that hold your head up on the bike.

Triceps dips: Start with your palms resting on the seat of a hard chair or bench (fingers forward), arms straight, and butt hanging in front of the chair. Using your shoulders and triceps, lower yourself until your elbows are bent at right angles, and push back up.

Neck curls: Use a strap/cord to hang a 2.5 lb weight from your helmet straps. Sit with the helmet on your head. Bend forward with your elbows on your knees, looking at the floor. Slowly hyper-extend your neck until you are looking straight forward at the wall in front of you, hold for 30 seconds, then slowly lower your head until you are looking at the floor.