¿Why are you covering 23 km a day? ¿Are you commuting?
Conmmuting 23 km a day can be done perfectly fine if you keep the effort reasonable.
I've been part of a bike delivery group, and as such, I had to cover 60 km a day, hauling 20 kg aprox of parcels. I don´t know how much did the racks and other gear weighed.
I never got a knee pain out of that, but a few of my mates did. They where all younger (low 20) than me (low 30's) except for one. My assesment is that they where pedalling too vigorously all the time. One of them used to brag he could carry the biggest loads using the heavies bike. The other one was a youn man getting in love with his fixie. They both loved to tell all others how fast they where, etc... These two that I mention where injuried bad enough that they bot got medical prohibition to use the bike for a time in order to heal. The rest of messengers who kept theyir loads whithing limits and had reasonable travel (and delivery) times, never complained about knee pain related to the job.
In my case, I never experienced injury due to overwork, just the expected tiredness. I was not a novice when I started, but came from a time of almost complete inactivity. I think the experiendce I had as a MTB rider (and a few multi-day long road rides) helped me a lot. The following points are the ones I consider most important:
Extensive use of gear shifts: I was doing delivery on a flat city, nonetheless i used a lot of shifting. Used easier gears to accellerate and only changed to harder gears when I got speed. AS others mention, keep a good cadence, never too low. Using harder gears in low cadence puts a lot of strain on knees and hip joints.
Good bike fit: Adjusting saddle height, and handlebar relative position is crucial. Having the correct height allows for more efficient tranfer of power, overall you'll feel less tired for a given travelled distance, less sore muscles and less joint fatigue. Some bikes may have trouble keeping the saddle height, so constant checking is a must (or solve the problem once for all). In my case the bike had some cheap components so I had to check twice a week until I upgraded the seatpost clamp.
Your body is not a machine: Some days you'll need to take it easy. If you are commuting, consider starting with only some days of bike commuting (use public transportation or car one or two days a week). As your body gets accustomed to the effor add more biking days until you reach the fitness level for commuting all week with the bike. Another alternative is that you take more time to travel on a couple of days, so you don't have to pedal at a hard pace.
And in general, observe good bike maintenance practices: Keep tires properly inflated, transmission gear clean and properly lubricated. Brakes properly adjusted, etc.