Unless there is something about your health that you aren't telling, a mile or two is not going to cause you any issues that you'll have to endure for a lifetime.

You probably shouldn't try to accelerate like a bat out of hell when when the light turns green. But regardless, you'll get plenty of notice from your body before you have a life altering permanent issue.

Sore muscles will be par for the course until your body gets use to the new exertion. Joint pain that hangs around for days should be investigated, both you by a doctor, and your bike to determine what to change. But if you aren't having joint pains, then don't worry about them.

Measure how long the seat post is and you can then look online to see if there are longer ones or not. Or if it has a model or part number, then you can search for it's specs.