Had to smile, when I read the "HIIT"...
With my sport career in swimming, the last three years' training, 1955-1958, featured what was then called "repeats"...
In those days, training commonly consisted of swimming "laps"... usually a mile of steady, non stop working out. This was followed by a time trial... a timed swim of one's competition distance. (in my case 200 yd. backstroke.).
My coach was one of two who "broke the mold" by changing to what is now interval training. It meant swimming full race distance at top possible speed, for 10 "repeats"... with a two minute break, in between. While this is now the standard, at the time, it was radical, and employed only by my school, and Ohio State which was #1 or #2 in the NCAA's.
Now, not so much... Walking isn't fun, 'cuz I walk like a duck and the pounding makes the joints hurt. In reasonable weather, I still bike 5 to 15 miles/day, and canoe. Recently bought an old Vitamaster exercise bike, and do a half hour in the AM, and a half hour in the PM... while watching TV... No stress, no strain, and very easy @13 m.p.h. Definitely not HIIT!