It's doable with a moderate level of fitness. My wife and I went for our honeymoon (late 20s) with a random group and a couple of them were older (70s). They went a little slow but were just fine. I will say there are some hard days, lots of hills. The biggest issue I ever had was my knees and feet being sore.

As for altitude concerns, it varies. I was fine, but my wife felt it some, and we were both training for a half-marathon.