Did you use a formula to determine your max HR?
50%-75% is a very broad range. Some people who are used to doing cardio might want to cheat a little bit by making it 70%-75% - they'll always end up at the upper end of the range because if they go lower it doesn't feel like working out to them. I'd say that a constant activity *averaging* at 70% performed for several hours is a bit too much - IMO it's ok for 1 hour per day though, or for 4-5 days per week. If you have the time you should extend the hours though and at the same time take the intensity down to 60%-65%.
Chronic Cardio for me personally involves heart rates of 80% max or more, and at least 45 minutes duration, performed several times a week.