Honestly you are thinking about the game really well for being level 24, I used to main Riven til Yasuo came out and she is a great champ for you to main because she has a lot of "small in depth things" that you can learn and exploit.
For extremely unfavorable matchups, Jax, Garen, Renekton to name a few are really tough for Riven because they all have cc that basically makes Riven useless while they sit on your face. The best way to beat Riven is to rape her senselessly in the early stages of the game deeming her useless for the rest of the game. Riven is one of the best last hitters in the game because of her passive so if you have to sit tower you really don't lose out on much because you can easily balance out your abilities in order to get good last hits under turret.
Unfortunately as Riven if you do get behind, it is really hard to come back because she is a champion that HAS to build damn near full damage in order to be useful at all, so farming is your best friend when behind.
I would never rely on your jungler to give you a start, because if you do that your giving yourself someone to blame other than yourself if you die at level 2 making stupid trades haha.
As far as rune pages go, flat AD quints and reds, armor yellows, and mr blues are prolly your best bet. you can try running some cdr runes in your blues but I personally just stick to mr, I haven't played as much Riven as I used to.
Since you are level 24 there are some small things you probably have no idea about. Try these in a bot game cause they will really help:
Riven can cancel almost all of you animations with some sort of other animation per say. For example watch Riven's ult animation. It is extremely long, super cool, but super annoying if you want to quick ult trade and merk your opponent. However, if you cast your stun right after you turn on your ult, it cancels almost the entire animation allowing you to start dealing damage a solid second and maybe more earlier. Now, your stun also has a fairly irritating animation that can also be canceled with 1 q cast. So now, you have full passive stacks and can really start dealing damage.
You can slightly cancel the q animation Riven has by sending movement commands immediately after using q. Try this on jungle camps in a bot game and it will help you a lot. The mechanic works like this: q, click right next to a creep to issue the move command, and then click the creep to issue an attack command. If you get good at doing this quickly you can get off auto attacks extremely fast. I'm sure there are youtube videos you can look up, I'd find one for you but I'm late for class and have to shower. Anymore questions just ask and I'll respond as I can. Glad I can help.