Don't underestimate how tiring it can be to ride a horse! A lot of muscles are at work, and I bet after a 70 minute horseback ride you'd feel it even more than biking.... I think it's hard not because it's particularly calorie burning, but because the muscles in your legs and back are needing to be flexed without being able to move and stretch much. You get really stiff.
Yeah, I've ridden horses and stuff, it is hard and you do get stiff. But at least you don't have to stare at a hill and wonder "how the heck am I getting up that?"
And I fought my seat almost the whole time - it kept slipping down three inches and I just couldn't get it tight enough. That makes things way harder. Once I got it fixed I did great.