Eldery Kangaroos get pretty big, intimidating coz their toenails etc are dangerous. You won't outrun them obviously. People have been killed by them. Generally though they'll run if given the chance. Don't fuck with Skippy.
When I was a much younger guy I was into hunting. I did a stint outback NSW, Bendemere, over the Moonbi ranges from Tamworth, working on a farm. I had use of all the farm gear plus guns so obviously went chasing big roos. Oh boy, the Greys were large, but they recognised a rifle and I could not get near them. They hightailed at first sight, hopping over fences like they were flying. The Wallabies were not so lucky, but I only potted one for stew and dog tucker. They were some pretty colored wallabie, forgot their name. 1970's.
Is it just my imagination or are the older bigger ones the first to run? More experience and more cautious? Lucky coz they're the ones capable of inflicting the most damage.