Was the walk-run-jog September 15? Were you able to participate?
Even though I walk 6 miles a day at 4.10 miles per hour, I am sure I could not have done the 5K beach walk, unless,
maybe I got to walk it all on compacted, wet sand. Walking in dry sand is a killer to me. It uses muscles in a way that mine here in the ocean-free mid-continent never get used. When I visit my Florida in-laws I always feel embarrassed at how exhausted I get just walking across the dry sand to the shore edge!

Nobody else mentions the difficulty, though, so maybe it is just me. Heck, all we have is mud along our Midwest river and lake shores. A friend of mine got dangerously near being swallowed up in riverside quicksand (quick-MUD!) this summer. Moving through that requires different muscles, too!
