Will it be hard to get back into this game?

Just "came back" from a 2 year break or so and they reverted adc item changes, so it's the same as when I left.
You should get used to some item changes and the new runes, which is not as hard as it used to be imo and go through new champion abilities real quick, so that you have an idea bout what their abilities are.
Took me about 2 weeks, which is not that much if you ask me.