I found 2 exercises made a huge difference in my ability to ride without fatigue. They're both hitting the same area so you can do either one. Romanian deadlift and back hyper-extension. You can buy a kettle bell or some dumbbells for the deadlift and do them at home. If you belong to a gym, the hyper-extension allows for greater isolation but they both work great. Start light and do 20 reps a day for a couple of weeks. I found it not only made riding easier but also improved my posture.
I'm never tired, my wrists never hurt, my back is strong enough stay low and move side to side without issues. Interestingly, I my fitbit records my rides as cardio.