I weigh the same as you I'm 50 and 5'10 I'd glad to know we all don't need to be built like Joe MarlerI'm almost 65. Only 67kg but I'm still well able to work with the SLX 22. Though I now only use it for gfs and conservatories. I use an Extreme 22 for maintenance washes.
It's not all about strength. Efficient technique, grit and determination is every bit as important.
One thing i try to remember to do is swap hands so i can even out which side gets the most work. I do keep forgetting to do that though!