I did something similar recently. Drilled the sides of the channel and chiselled out about 75mm deep. Took a few hours but my shoulder still hurts if I scratch my back!
It’s tough and does depend how strong the concrete is but stitch drilling and sharp chisel helps. Also not hitting your hand with the hammer reduces the pain involved. Wear knee pads too if possible.
I did something similar recently. Drilled the sides of the channel and chiselled out about 75mm deep. Took a few hours but my shoulder still hurts if I scratch my back!