The trick with children of this age is to be one jump ahead! If you know something is going to pull their chain, get in with the solution before it happens!
"I am just getting a biscuit to keep you going when we are out - just pop in the pushchair please and I will go and fetch it." [I know, I know it is bribery by another name, but it works! You know he is going to have a melt down about it so do what you need to do to stop it before it happens. Before is the important word here!]
Build in incentives. "I am going to see how far I can push you up this hill before I start to puff and pant. I bet you couldn't do it!" Or - forget trying to get him to walk it at the moment - minimizing the possibilities of meltdown could be the way to go.
He was probably hitting his sister because he was overwrought about the things that had happened when you were out with him. Once a child of this age gets in a state it can spill over into lots of bad behaviour that seems unrelated to what triggered it.
Unfortunately children, even very small ones, do pick up in anxiety so you may be getting into a vicious circle here.
Might it help to write down the things that trigger his meltdowns? That way you can have strategies ready to try and prevent them happening. It will help you to feel more in control - feeling out of control is very anxiety-provoking; and basically the more your child feels you are in control the more he will feel secure, which has to be a good thing as it might reduce the bad behaviour.
One example from my own life is that one DD used to be a total pain in the supermarket - but the solution was to tell her that when all the shopping is finished and if she has been good she can have a ride on the sit-on deer that was at the exit.
Basically his behaviour is entirely normal, so do not worry on that score. And everything I have suggested is very easy for me to say when I am not in the middle of it! - I do understand that. But I have been there and know how it feels; and have had more than a bit of practice! - in getting it right and in getting it wrong! I hope that some of these ideas will feel helpful.
I know that some people will feel the element of bribery is unacceptable; but another way of looking at it is that you are giving the child the chance to behave well and the opportunity for you to then be able to give out a bit of praise - often a rare opportunity with a toddler!