Ahh, it's so great to talk Isle Royale routes. Seems like it's been forever!
Jesse, according to our handy mileage chart provided by the very-awesome MikeT (found here:
download/file.php?id=492), it say's it's just a tad under 17 miles. 17 miles is a lot of hiking on Isle Royale in one day, but it's doable...and has been done before by people in similar physical condition. Here's my two cents, though:
The climb out of Hatchet is a quad-killer. After that, the hike to the the Island Mine junction is pretty easy--the Greenstone trail there is like a super highway. It's on the inside of the island, so it'll be warm. Not much of anything terrain-wise, except a nice leisurely walk in the woods. Finally, the hike down to Siskiwit is just that...a hike DOWN HILL to lake level. It's also pretty easy (although there is one up hill that's a little brutal, but it's not bad at all).
Between hiking 12 miles on the Greenstone, then going downhill most of the time to Siskiwit, followed by a beautiful walk along Lake Superior for a few miles into Siskiwit campgrouund, I'd say this route is very do-able. Plus, you'll have the benefit of tossing in the towel at Island Mine should you decide the first 13 miles you just hiked are too much. So you have a built-in back-up plan should you need.
I'd say go for it. I have one word of advice, and one word of motivation to make the jaunt: first, budget 10-15 minutes to explore around Island Mine after the Island Mine campground. There's some cool stuff there, including an old (giant) locomotive. Next, Siskiwit Bay campground is one of my favorites on the island. I always sleep good there because it's by the lake and it's always cool and the sounds of the waves are amazing. Lastly, the north-facing dock is a great place to lay and star-gaze at night.