i rode to Romney last year from Boston area, nearly 600 miles each way, by myself both ways. And it rained a good part of the way both coming and going. The ride there sucked. It was during the week and i hit a lot of traffic. Coming home on Sunday, I planned a different route, which was a bit longer, but more remote and it was a lovely ride.
I've done many long days in the saddle and my advice would be the obvious: pull over as soon as you feel even a little bit tired. If you drink coffeee/caffeine, have some - but not too much as this will make you have to stop and pee more. Gas up, grab a small bite to eat, walk around the parking lot, do some light stretching. In a 500 mile day, I'd probably make maybe 3 of these stops at 15-20 minutes each, plus a 30 minute lunch break. I like to take my time getting up and packing up on that last morning. A lot of guys feel the need to get up early and get the hell out of Dodge as soon as possible. And that's fine, too, but I just prefer to be relaxed and not stressed. The extra hour or two it takes me isn't going to matter. Daylight ends around 7:30-8pm in early May, so you have plenty of time if you don't like the dark. Even on mixed roads, averaging say 60mph, plus 90 minutes in breaks, is still < 10 hours. Leaving at 9am, you'd be home by 7pm, conservatively.
As was mentioned, the type of miles matter. Slabbing on the Interstate for hours on end is a good cure for insomnia. Plan a route that mixes things up a bit. Not only will this keep you from getting tired, but you will also enjoy the ride a whole lot more. I've ridden with you once or twice before, so I feel confident in telling you that you can certainly handle 500 miles in a day.
2014 DL650 (sold)