500km ride , 2 weeks before an HIM

I don't think it will surprise you that going from 70-90mpw to 300+ over 3 days is going to put some fatigue in your legs (deosn't matter how easy you ride).  2 weeks out from your HIM, it is almost certain to have an impact on your race.  But how great an impact is harder to say.  With 2 weeks in between, you should probably be able to recover OK by race day.  But that recovery will likely come at the expense of some race prep work.  Again, how big an impact that will actually have on your race result is not easy to pinpoint.  And whether that difference 'matters' to you is entirely personal.

If the 500km ride is something you really want to do and you are OK if that means you might struggle a bit more in the HIM, then go ahead with your plans.  Have fun at both events and enjoy the opportunities that your fitness provides you.  If you really want to give yourself the opportunity to do your absolute best at the HIM, then that's probably not the best way to spend your weekend and you should stick to training.

Agree in general with what has already been said.

But I will add that there is a lot more factors than just distance.  If I could hand pick 40 other people to ride with me, where we can ride 3-4 wide on really smooth roads, without any steep climbing, I'm pretty sure I could knock out 500 km in 3 days without causing any residual fatigue.  The draft benefit can be huge in a big group...so much so that sitting in the pack can save you over 50% of the effort that the guy(s) at the front are putting out.  I've done rides where we're cruising along at 25 mph, into a head wind (which I can't feel) where I'm barely in Z2.  The guys up front are likely at threshold though...and rotating...but you can literally sit at the back and get a free ride if the group is big enough and you don't get dropped on any hills or rollers.

On the other hand, if the group you're riding with is smaller, meaning you don't get as much of a draft benefit, and you have to do some share of the work...along with rough roads and climbs...then even a single 100 mile ride could be a lot to recover from.  The good news though is that biking is non impact, and recovery time is a lot less than say running 100 km in 3 days.