I googled the website for the marathon
http://www.saltlakecitymarathon.com/
and came across this helpful tidbit in a press release :

The results of an extensive weather analysis of 30 years of weather history for April, May and June shows us that June has a significantly better chance of clear skies and pleasant temperatures. The first Saturday in June shows average temperatures of 54°F at 7:00 a.m. for the marathon start and 70°F by 1:00 p.m. at course closure …..well within a comfortable range for participants.


Here's a map of the route. The good news is, the marathon route extends from SLC southward, while Wellsville and Logan are north of SLC.

Might make things a bit more complicated, though