I've never ridden a bike before, and I'm not in very good shape. I started riding yesterday (4/15/23). I think my legs are too short, or my seat is too high because the seat digs into the back of my thighs when I pedal. Also, as of today, my back hurts. I don't think it's my posture? It might be. I'm not too hunched over and the pain doesn't feel like that. It's sort of in my mid back and it hurts when I arch my back. Also, I know my wrists are bent pretty badly and I'm working on it. Thanks for the help :)
Your position on the trike looks mostly fine. You said the saddle digs the back of your thighs, that may be the saddle is too far forward, or it could be too wide for your bone structure, or it could be just getting used to that..
I'm not sure if your saddle is high enough - if your pedal is fully down then your knee should be almost straight but not quite. Another rule-of-thumb is your saddle should be just lower than the height that makes your hips wiggle while riding.
A too-low saddle tends to make one's knees sore, not thighs. The photo has no shoes, which will add another 10mm to the platform
It could also be a conditioning thing. Getting into riding takes time to build one's endurance. If you're riding longer than you're comfortable for on a bike, it could be a matter of riding slower and taking your time.
Another possibility is that you're on a trike, which holds itself at a right-angle to the surface of the ground. If the road is cambered this may make it slightly sloped, meaning you need to use muscles asymmetrically to hold your torso vertically. Other trike riders please chime in here.
But in the main I started riding yesterday you may be expecting too much too fast. Give it time, ride at least once every three days for a month, and see how you get on.
Don't expect too much too soon, and persevere. You can do this.
User Criggie suggests "if your pedal is fully down then your knee should be almost straight but not quite.". This is very true for a regular bike and some tricycles, but adjusting a trike bike can be a little more tricky. You are supposed to be at a quit low position always with knees bent, and I guess the saddle is far too high for you.
Watch this video at 10:16 to get a sense of how it works. This video can introduce you to proper adjustment. Also consider to take your trike to a shop for getting help to select a saddle optimal for your anatomy and riding style.
If you have no particular reason to choose a trike, then a regular bike might be a much better option for you. You can learn to ride it in a few hours.