Shorts are a personal preference, so any of mountain bike shorts or road shorts or road bibs are fine or pretty much any other shorts that you are comfortable wearing on the bike.
(I often wear football (soccer) shorts, 3/4 training pants on cooler/sunnier days, but that's just me, from my footballing days I've still got lots of perfectly good shorts and training pants and even though I've been off the pitch for a while, they work for me.)
Wear what you want to wear, but most importantly, wear what *you* are comfortable wearing, and wear you HAVE worn.
The day of your 70 mile ride is *NOT* the day to ride with a new pair of shorts for the first time, new gloves for the first time, new shoes for the first time.
It's also not the day to put on new pedals for the first time (doesn't matter if you are switching to flats, clips, or clippless), or switch out where you store your water (down tube mount, seat tube mount, triathlon mount), etc etc etc....
It's also not the day to try new foods or drinks.
Never eaten a peanut butter sandwich? Then don't. Never tried Skratch Labs products? Then don't. Never drank Gatorade? Then don't.
(But the best peanut butter sandwich I ever ate was in the rain in Marshfield Hills.)
You don't want to discover that something you ate or drank is giving you stomach cramps (or worse) while riding.
If it's a supported ride, you WILL find products you have never eaten/drank at the rest stops. Resist. But if you are tempted to try something that you've always wanted to try, you may take it, but pack it away and try it another day.
(That said, jorts are probably not the best choice for long rides.)
Good luck!
-mr. bill