I am a young 51.
At the moment, I am running 5 days a week. In the next two weeks, I plan on adding another running day. During the week, I try to get 13 - 15 miles in each run day. Saturday is my long run day and I like to get in at least 23 miles. Today I did 31.5 miles.
Congrats on your swimming. That is great progress, not to mention a long distance to swim.
In regards to knee problems, there is no peer reviewed evidence (at this point) that shows running a lot produces knee problems (re: arthritis etc). At the same time, if one has a family history of problems in this area, does not wear the proper shoes, does not practice good running form, and rest properly between runs, physical problems can most assuredly develop.
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Glad to read that you got your mystery solved and are on the right track.