If you compare that with Joe Average, who works 8 hours a day, 5 days a week, 46 weeks a year. That is the same as 1610 workingdays (of 8 hours, breaks and traveling time not included).
The Vegas seasons of 2-3 shows a day (eventhough that's 3x 1 hour working), 4 weeks in a set must have been terrible. But don't forget that after such a Vegas engagement, he was free for quite some weeks. Many Vegas performers (often the lesser stars) actually live in Vegas and see it as a jon (where you also work every day).
The tours also were very short: only 1-3 weeks per tour. Always in the same country (mostly even in the same part of the US). That's different from artists touring the world and being away from home for months.
So in short: the shows in 1969/1970 were very physical (and I guess also very demanding for Elvis' voice). But later on the average Elvis show wasn't a full-speed ahead, high energy, 'give it all' concert. Just watch the Aloha: I love this show, but Elvis is very, VERY static with little movement. If you watch fan footage of Elvis shows, Elvis does very little else than walking around during singing. The amount of karate chops are limited to just a few mostly at the end of a song. Even songs like Polk Salad Annie and Suspicious Minds were shortened to save on the karate routine.
Don't forget, Elvis was ONLY in his late 30s when the energy level of the show dramatically started to decrease. I don't like to compare Elvis with other artists (since he's a league of his own), but for the sake of discussion I will do so: Brian Johnson is the leadsinger from AC/DC. This guy is 63 recently came back from a two year long world tour. These shows were about two hour long where the whole band gave 100% to the (huge) crowds. Same goes for the members of Iron Maiden: all in their 50s and still giving amazing shows around the world. Or Mike Love (from the Beach Boys): his performance (movement and stance on the podium) can be compared with Elvis (unlike singers from hard rock bands of course). He'll become 70 this year and he (along with fellow Beach Boy Bruce Johnston) has about 100+ shows a year. Shows from about 80-100 minutes.
I don't want to put Elvis or his shows down. I only want to make a point that it wasn't the amount of shows or the energy Elvis put into them, that brought Elvis down. Elvis lost passion in things quickly: making movies, recording, performing, women, food, etc. He was clearly out of balance. Just notice that Elvis did very little in all these weeks (sometimes months!) off that he had (after movies, tours or Vegas engagements). He didn't take acting classes, his weight often increased, he didn't try to write music himself, had no interest in the business behind show business and didn't growas an artist.
Just like with making movies, Elvis started strong in 1969-1970, but then it all became a drag and a way to make money and an attempt to fill that void in his life. But it wasn't pure passion anymore that made Elvis tour, extend his Vegas contract (or visit the studio) during the later part of the 70s.