That is pretty much exactly what I expected!
Even so, I expect by scouting out the route really thoroughly you could escape the bonus miles. What size tires were you using?
Yes - more thorough scouting and being intimately familiar with the entire route would be a pre-requisite to ever attempting to do it in 24 hours. There were places we rode where some serious lights would be needed at night just to avoid getting stabbed in the face with branches that were overhanging the trail. Some parts are clearly more well travelled than others.
I was on a cyclocross bike with 28's, and my buddy was on his road bike with 25s. Made keeping speed through the muddier parts quite interesting. He has a gravel bike which would have been a better choice but wanted saddle time on his road bike before heading to RAGBRAI soon.
I think the 360 in 24 would be possible, just not by me!