1. The Chicago Urbanathon is an obstacle type race, therefore it is more like running a series of longer intervals, not 10 straight miles.
2. Appropriate volume of squatting isn't going to impact distance running much, once you get used to it. Although the distance running is going to slow down the rate of improvement in the squat. The best distance runners in the world just run (Kenyans), however, I would suggest that Eric is NOT one of the best distance runners in the world, rather, he is looking at keeping a more balanced range of abilities and is seeking an intelligent way to do so. Low volume strength training, with moderated intensity is generally compatible with running training.
In Eric's case, setting up some sort of more specific training on a bi-weekly basis might help...running 1-2 miles, then engaging in some strenuous work to simulate hitting an obstacle course would give him an idea on how that kind of work will impact his running in between.
It could be something as easily set up on a track:
1-2 mile run, 5 minutes of stadium sprints
1-2 mile run, kettlbells swing/burpee combo
1-2 mile run, push ups and pull ups
1-2 mile run, shuttle run
Just some shit like that.
As far as the squat training, what's most important in the race is the running, since that is going to take up the most time.
Linear progression while training that much running might be too much to ask for, unless he's very undertrained in the squat.
I would move to a more intuitive weight selection.
Warm up, then
Set 1: 2-3 reps left in the tank
Set 2: 2-3 reps left in the tank
Set 3: 1-2 reps left in the tank