Let me begin by recommending you don't do this ride unless you're pretty confident in your fitness. Better yet, don't do it unless other people are pretty confident in your fitness. By the end of the 45-50 miles you'll be plenty tired from the constant up and down nature of Mana Road.
Having gotten that out of the way, the Mana Road ride is absolutely beautiful and worth the ride. It's fire road the entire way, absolutely zero singletrack. Although it's not technical there are plenty of opportunities to slip in the loose rock and end up with some serious road rash, especially at the speeds you'll be going on the downhills!
The start of the dirt road is marked on the Google Map directions on this page. You'll need somebody to drop you off at the start and pick you up at the end. Plan on this ride taking all day, it probably won't, but plan on it anyways by starting early. At points you'll stop and think you must be close to the end, then you look at your GPS and you're only 20 miles in, not even halfway.
Also it's pretty cold at this elevation. Bring warm clothes, a rain jacket, plenty of water and food and any other supplies you would need for an all day adventure that could go awry.
And remember... have fun!
PS. The last 11 miles are 100% downhill :)
Clicking on the link above you bring you to Google Maps where directions to the trail are plotted in from Honolulu Airport.
Download the GPX file and use your favorite software to upload trail data to your GPS. Right click on the link and select "Save Link As..." in order to download it.