I know this myth is false first hand! Frequency of exercise is way more important than duration. You can't get a weeks' worth of workouts done in one or two days. Sorry weekend warriors!
When I was in high school I would commonly workout twice per day. I'd be at rowing practice in the mornings and then after school I would 6 km run to the gym and lift weights. I was burning myself out completely. When I came home it was game on: me vs. the fridge, and I always won!
I think that my experience explains the problem that many people have when they exercise for too long. When you rapidly deplete your body's energy stores it feels like you need to eat a lot to replenish yourself, in my case a lot of ice cream. The truth is you don't need to eat as much as you think you do and it's totally a mental thing. I learned that the hard way! You're hungry, you've worked hard but probably not as hard as you think and so the dangerous cycle of reward eating begins.
Even if you aren't a reward eater working out for too long isn't as effective as you might think. The truth is you just can't work as hard after 60+ minutes so unless you're training for a marathon or something equivalent there really isn't a reason why you should exercise for longer than an hour. The reason for this is because when you workout for long periods of time your body essentially goes into cruise control mode. It's tough to shock your system when you're essentially running on auto pilot.
The best thing for your body is to do 30-45 minutes of intense exercise 4-6 days per week. You have the time!
Think of it this way, if you brushed your teeth for 30 minutes on Sunday would you be good to go for the rest of the week? No. Same goes for fitness.
Bottom Line: Every workout counts!