Since I've learned the hard way that Garmin Connect is rather unforgiving: I cannot Add a Race Event *AFTER* the fact to my calendar and link it to the activity.. This time I was smart... I'm going to beat this ConnectIQ and somehow get it to work for me. Or so I thought...
I looked up in advance the "City 5K Walk & Run (5 km)" last week. I added the event to my calendar. I did this even this morning (great run!).
Now I've got you ConnectIQ! This time ConnectIQ can't beat me with it's little Gotcha tricks! Hah hah hah...
I ran the 5K. I recorded my activity on my watch. I go into my ConnectIQ calendar. I see the event in my calendar. Oh now I can smell victory over ConnectIQ!
"We Couldn't Find Any Running Activities on MM DD To Link To This Event"
Nooooo.... WTF! I was there! I did it right! Grrrr... foiled again. ConnectIQ reached up and snatched my victory with another one if it's "Gotcha" plays!
Turns out the Verified Source Event Type was coded as a "Walking" event (it was both). I ran the 5K and recorded the event as a run.
Walking events and running activities? "Never the Twain Shall Meet" in the world of Garmin ConnectIQ.
Nicely played Garmin. Nicely played.
I bow to your never ending depths of gotchas. 
(I also tried connecting in the other direction from the activity to the event. And tried it both on the app and the web. No joy)
So again the whole system of linking calendar events to activities is unforgiving with 'gotchas' that can't be overcome by a user.
You cannot go in after the fact and add a past event to a calendar and link it to a running activity.
You cannot link an event to an activity if the event (created by someone else) was incorrectly coded as say Walking vs Running.
