Actually you are right where you should be given the facts.

First, not sure what type of bike you have. A road bike can be much lighter with clip ins.

Second, especially for road bikes you should get a fitting shortly. We all have different flexibility and you want to be sure the Bike is dialed to you. Can't say enough how important a good fitting is. Bike tech gets you to half inches or so and good fitting to the millimeter.

Third it just takes time and miles for your hands, arms, back, legs etc to adjust to cycling. Days wont do it, while months can be slower too.

Fourth, need to build up your heart lung and your legs. Like others have said it just takes time. Someone said 2,000 miles. Dunno did around 1,200 first year starting at 65, now almost 71 and rode about over 6,000 last year.

Make it fun, cuz it is!