take a ride on the river with a GPS and you'll know. SK48 did that the very first year and did report on the lenght. I can't remember what it was but it was less then 5 and more then 4.
Seems to me it was about 4.4-4.5 miles... I made 35 laps (damn battery bracket broke on lap 10 and I lost about 45 minutes... It also didn't help starting 50 minutes after the eventual winner...) and I think the GPS was around 160mi or so... I don't remember exactly and it's long since been reset, but I think that's close... We were running mid-high 70's and low 80's pretty much the entire race, and my lap times show about a 75 mph average if it was five miles...