This is the general question:
How often and how long should do soft aerobic exercise a middle-aged man in his 40s, provided that he his healthy but out of shape? If possible, please focus your answer specifically on fast walking as exercise
Now, after the general question, I would like to add some particular details that may help the readers understand better what I want to know:
I am 41, quite out of shape but I have permission from the doctors to do aerobic exercise. Because several months ago I returned to weight training improperly after 18 years of sedentary computer work, and then aggravated it by improperly stretching my pecs and carrying my 16 kg doughter always on the same arm, I am still recovering from several tendonitis in both shoulders.
In the meantime, I am doing some aerobic exercise every day. But, after having injured myself so badly, this time I want to be sure that I am doing the aerobic exercise properly and that I am not going to die suddenly in the middle of the street because I was stressing my heart too much, or who knows what may happen this time...
This is exactly what I have been doing in the last 11 days:
I start walking very fast, just at a pace where it seems that I should instead start running. But I don't run, I just stay walking. I do this uninterruptedly for up to 45 min. I wear a fairly good running shoes. I check my pulse at least twice during the walk, and it always stays between 120 and 150 bpm (I never let it go over 14 beats in six seconds). I breath correctly. I don't feel it is exhausting, but rather slightly challenging. I think I could say some sentences aloud while walking, as long as they were short and there were enough pauses to breathe in between. I sweat a bit but not too much. At the end, I stretch my quads a bit.
I have written down how long I do this every day. This are the times up to yesterday: 25, 30, 36, 45, 45, 41, 45, 45, 43, 42 and 45 min.
Is that wrong? Should I insert rest days or something?
If this question seems too localized because of the additional details I gave, please feel free to answer only to the general question at the beginning, and I will try to derive the appropriate adaptation to my personal circumstances. Nevertheless, I think that explaining the correct way of starting a soft aerobic walking program for middle-aged unconditioned people is something of a general interest, and the personal details I added are valuable as an example of how it should (or how it should not) be done.