His feet are sinking in the sand with every step and he can't remember when it was so hard to simply walk down the beach. It's different when you're little; there's less gravity to weigh you down. And maybe it's also different when you're not tired or sporting a wide variety of bruises - in that instance, he doesn't have the experiences to be able to tell. But he's intent on making up for those lost moments because as of now, he's a spy no more.
He sees something move behind the tiny windows of the shack and has to assume it's Sherlock. Be it far from him not to see him coming, really. Mycroft Holmes has never been able to hide anything from his little brother. Not for long, anyway. By the time the kettle blows, Mycroft is glad to step out of the sand and onto the front porch.
Kicks the sand out of his shoes, tugs at his coat to make sure that, in spite of the blood spatters on it, he looks decent enough.