Yeah, Muay Thai. It'll be way easier to stay motivated to train, especially if your alternative is to run twice a week. Won't make you big, but it will make you tough. Start rolling those shins!
You need to do MT at least twice a week to progress properly. If you're too sore to do 2 days MT and 2 days weights at first, don't worry. Your body will adjust. When you start any martial art from scratch, your body will need to get used to doing stuff it's never done.
When I'm not injured (which is not right now
) I try to train 4 days a week at the kickboxing gym and 2 days doing other stuff, like running, bodyweight exercises and some free weights. I'm not trying to get big though. I'm trying to keep my weight where it is.