...as the name states...
I found that I was not getting much of a challenge swimming laps; it got boring and the weekend use in summer leaves too many contaminants in the water (not just chlorine...:yack)
--When I switched to roughwater swimming I discovered a whole new world; tides, surf zones, changing conditions, dolphins, seals, fish
and much, much more!
--A good two-hour swim in the ocean 4x a week leaves the body feeling great and the heart, mind, and soul renewed!
--As far as heart-rates are concerned, feeling which way the current is running and swimming against it will guarantee you'll hit the % mark you seek.
--That, and swimming when there's plenty of chop/swell. If the current is really moving, it is possible to get quite tired in 45 minutes; if it's calm, 2 miles or 2 hours.
Ultimately?
Find a sport or exercise you love and go for it.
A nice alternate for those 'off days' is a workout w/weights. Gotta keep the calcium on them bones. Plus, the added muscle mass increases your metabolic rate.
Nice thread, BlueIcarus.
If running is what makes you feel better after a session, then maybe your body needs that, and it is the right discipline for you. Wear good shoes, be good to your knees...
most of all, enjoy.
See you in the water!