"the hardest bit was putting the tyre back on, as the more of it I had pushed back under the rim, the tighter it became."
...as I wrote, if you had used talc it would have slipped on easily. I find I don't usually need tyre levers to replace a well-dusted tyre.
The sore sit bones are at the bottom of your pelvis and they are what take the majority of your weight. A small amount of weight should also be taken on your perineum, which is the bit between the legs. In order to achieve a comfortable sitting position you will need to experiment with the height and position and posture of the saddle - it can be raised and lowered, slid back and forth and the angle of cant adjusted. For most blokes a slightly nose-up posture seems to work best.
You are using padded shorts, worn next to the skin, I hope? A bike saddle will make you sore if you are wearing just trousers.
