@Amanda_W There are a lot of variables and missing information related to your query. You stated length of ride, some hilly terrain, some (all?) gravel surface, using a road bike on gravel (not a problem I do it all the time on 25mm tires at 100psi), and that you had been working out with weights 2 days prior to your ride (generally not an issue).
There is nothing wrong with tips given already, but I know (and
@Dusan ,
@GirishH , et al.) I can provide indepth advice from experience having done long (75 to 100 miles) and longer distances than your recent ride. You are welcome to PM me as I tend to be longer winded than most and I like to cover a topic as completely as possible in one shot.
I will say this unless you have done a serious ride (not sightseeing or with frequent stops) of over ~4hrs before it is hard to equate that with doing a couples hours or less than 50 miles or so (as you probably now know). Mind and body are at a different level altogether, and even with experience at shorter times and distances the same experirnce does not pertain as much as one might think once you have your butt in the saddle for half a day or longer.
If you have ever done a long nonstop (except for quick fuel/restroom stops) drive in a car you know the difference; especially if the route is without much environment change. I am talking about the difference between a 3-5hr drive and 10-20hr or more drive (I routinely drive +20hrs nonstop, carrying my own food, extra fuel, and means for urination while driving). They are 2 different beasts altogether and your body and mind need to be prepared in an entirely different way. Even most truckers do not what I do; nor do I believe they are supposed to for safety reasons.
I regularly do 4-5 hour rides covering 40 to 50 miles and a little more, but that includes a couple extended hydration/stretching breaks (for me about 5 munutes each) and a prolonged relaxation break (30-60 minutes) for resting, eating, hiking/sightseeing, etc.
Those rides, although a good preparatory start for mind and body conditioning, are nothing like doing a true 50 miler (only the water and stretching breaks if even that); and that is nothing like doing 1.5 to twice the distance in the same manner; although it goes a lot further regarding the preparation and conditioning required for the longer rides. As a solo rider, and without anything relating to entertainment/distraction (no riding buddy, no music, etc.) I have honed the skills needed to face the mental trials of being on the road and essentially (if I have my way) being sequestered from civilization. Night riding takes that up to the next level since you really only have the pavement or dirt right in front of you for any visual stimulation. Even a short night ride can be mentally taxing on those unprepared for it aside from the extra inherent danger involved.
That was me being "succinct"!