The map at the link says:
Salt Lake police officers will allow vehicle traffic to cross the marathon routes at major intersections as race crowds allow, according to the race website.
If you’re coming from the north, I’d get off I15 at 6th South, by about 9:30, and try to cross 9th east at 4th south, or 11th east at 9th south. You might have a little bit of a wait, but I would think you’d get though wit plenty of time to spare.
In the past, I’ve been able to drive parrallel to the route a few blocks, find an intersection that opens periodically, and get through without too long a wait.
Good luck!
I’ll be there as usual, but can walk from my house and not have to cross the marathon route.