I have followed several long distance walkers, one on a blog and an other in the book they published about it, as well as seeing blog items that I do not follow long term.
They use wheeled transports, dragged behind them, for parts of their trip or even for the whole.
Here is a link to the blog I followed. Only a small part is in English, but it is enough to show what he used. Jan Vroomans walked from the Netherlands, with no destination in mind, towards the east and decided it was enough when he was in China. Early on, maybe even from the start, he used a two wheeled luggage, at this time I can not find it on internet, the nearest I can find is a 'Burley travoy urban bike trailer', which might do the job.
One of the others used a more conventional hand kart which he dragged behind him, gripping it with one hand. And he only used it for the areas where re-supply would be very hard, like in the USA deserts.
I would say, having done it, that a 15 kg backpack is within reason for carrying, if on the heavy side. People I met traveling were carrying 30 kg backpacks, without training and out of an office job, they did suffer from back and knee complaints a few days in.