So, I received my $15 digital counter from Amazon today.
Mounted the magnet to the back of the wheel with double sided tape and the magnetic sensor to the bearing support, also with DS tape.
The counter runs off a single AAA cell and that's now stuck to the front of the enclosure with Velcro.
Each revolution of the wheel will produce one count. Since the internal diameter of the wheel is 12 inches that means the internal circumference is 37.6991 inches or just over 3 feet.
There is 5280 feet in a mile... so if he runs around the wheel 1,760 times he will have run well over one mile.
Will be interesting to see what happens. I'll report back with the results and will give you just the count so you can convert to meters, if you wish.