When I'm hiking, I hike all day long, but when I do cardio at home there's no way I could do two sessions in a day. I'd either be too exhausted to exercise the next day, or I'd cripple myself with over-use injuries. Even in my 30's and 40's, when I was running 21 miles a week, that limit was set by what I could do without getting overuse injuries. Every time I tried to build up beyond that, I injured myself and would be unable to jog at all for weeks. My goal right now is to get to running 3 miles two days in a row, then a day on the exercycle, then two more days of running 3 miles, then a day off. I'm not there yet, but I think if I get there that's about the best I could hope to maintain. Oh, and two or three days a week of very moderate resistance.