You need to change your mindset from thinking about distance, and instead think about time.
Car drivers don’t appreciate the distinction, but, if you can ride easy at 10 mph, you are looking at 7 hours riding time, so you need to think about what you would normally eat and drink during that time, and probably double the fluids.
If you are not used to riding 20-30 miles once or twice a week, I would get some practice, not speedwise, just for being sat on a cycle for an hour and a half at a time.
You would maybe expect your legs to hurt a little, but you may be surprised to find that over 70 miles and not used to it, your arms and neck will most likely suffer too, and the neck pain might cause you a headache if you are susceptable to such things.
Wear cycling gloves, think about a hat with a peak, crash hats make you very hot indeed, meaning you will need more fluids, I personally would think twice about the crash hat, especially if you are not fit.
I know the arguments for and against crash hats, but a newbie in heat and not too fit may find the extra demand of overheating too great.
Depending upon the terrain, a newbie should budget around 9 hours in total including stops, more if there are significant hills.
Do not be decieved by the number of gears available on your machine, you should be pedalling around 50-60 rpm (fitter riders will pedal around 80 rpm when time trialling and up to 140 rpm when in a chase of a breakaway), and change gear downwards until you can find one easy enough to pedal at that rate, newbies often try to ‘save’ gears for when they think they will really need them by riding in too big a gear, your pedalling rate should be fairly constant and the gears determine your speed - pedal faster means going to a higher gear pedal slower means go to a lower gear.
If you don’t follow this advice, you will be blown out within 40 miles.
If you are likely to get a lot of direct sun, you may need to think about heatstroke and sunburn protection, always carry some plain water with you as well as plenty of other fluids as the plain water is good for washing flies out of eyes and rinsing cuts and grazes.