This is valuable information for me, I've always had an audio book or a podcast playing while taking walks. What happens if you row for longer than 20 minutes? Do you notice symptoms if you break a sweat or get physically tired?

I'll try walking with earplugs today.

You may absolutely have a point. This far I've felt limited in the amount of physical activity I can take overall and I haven't even considered doing something else because something as easy as walking has been making me tired. The idea behind walking for a specific length of time has been that I want to do as much physical excercise as possible without getting exhausted, and I figured that increasing the time I was out walking by a little bit as I managed the previous "level" would be a nice way of slowly ramping up the challenge. But if I notice that I'm able to do other things aswell, you may just be right.

EDIT: Also, I've been thinking about exercising with heart rate in mind after seeing a video describing the Boston Protocol (I think this was the name). In the video, a man's heartrate was monitored, and the man was told to inform the observer when he could feel symptoms returning. Then, the man worked out a program with a personal trainer, putting exercise intensity at a level that would place his heart rate at 10-5% below his heart rate at which he could feel symptoms returning.
Have you looked into this approach, tried it and if so, what did you think of it? If not, what is your reasoning around this?