I can only offer my experience...you mentioned your age, so I'll just say I was several years older than you when I built my WFO, that was the last time I did a couple of fairly large sessions with bagged mix.
When I built my
wood-fired oven, I used 80-lb bags of Quikrete. I poured two slabs, one on grade that the base/foundation of the oven was built upon, the other the hearth slab (poured on top of the base) upon which I built the oven dome.
Each slab was in the vicinity of a 40 bag pour, the hearth slab being the larger of the two at 44 bags. Both mix sessions involved mix for the slab plus filling some CMU cores. I mixed by hand using a mason's hoe and a wheelbarrow. The slab on grade was easy, I mixed then just dumped the mix in the form. The hearth slab was a little more work, I mixed in the wheelbarrow but then transferred the mix into drywall buckets and dumped the mix into the raised form, which was about elbow height. I'd mix two bags at a time in the wheelbarrow, each slab took around 2 hours to mix and place.
When I work, I generally don't screw around. I just go at it.
If you do attack it by hand, I'll offer that it takes me a few batches of mix to get into "zen mode". The first few batches of mix I'm a bit clumsy and my movements are inefficient. I get my first batch mixing, and while mixing, I drop the water hose with a nozzle on the end that has enough trickle flow to to fill a drywall bucket with water for the subsequent batch while I'm mixing the previous batch. Enough flow to fill the bucket but not so much to overflow and flood my workspace.
When I'm mixing, I don't use all arm motion. Once I get going I hold the mixing hoe in hands with my arms 'sort of' locked at my side and rock my entire body left-right with my legs. It's my body motion that creates most of the back and forth mixing action for the hoe, not my arms, and that keeps my arms from tiring out.
When the batch is mixed I place the batch in the form, then without cleaning the wheelbarrow I slice and and dump a bag of Quikrete into the wheelbarrow, then add water from the filled drywall bucket, when that's together I'll add a second bag of Quikerete, add more water, and finish that. While I'm mixing that batch, the water hose on trickle is filling the drywall bucket for the next batch. It's a non-stop cycle.
I find that putting the hoe down and picking up the hose and hand spraying water into the wheelbarrow to be irritating versus restful. It slows my workflow and allows my mind to wander and think about the work that that lays ahead. For me, standing still is bad. It drags the project out. I like music playing. I'll usually stream something. I'll pace my work to the length of a song, along the idea that each batch from starting the mix to placing it in the form should take say two songs for example. Keeps my mind occupied and keeps the pace of work going, and offers me a mental challenge for each batch.
In the end, figure out what works for you. Regardless, don't sell yourself short. When you start, if you're like me, give yourself a bit of grace while you figure out how your body (and mind) work.
