I've been thinking about your post as I was biking to work this morning. I see others have talked to you about tires in the meantime but for on road riding I can really recommend pretty slick tires even on a mountain bike they will make a huge difference in resistance, especially if you inflate them as hard as they can take(look on the side of te tire for something that says inflate to xx psi). Hard smooth tires run nearly as good as racing tubes.

Another thing I was thinking about is saddle height. Most new cyclists put their saddle way too low, which is understandable as you want to be able to put your foot on the ground when you lose balance. However that is not the best thing for transferring muscle power to your pedals. I found that for good force you want to have your saddle so high that when you sit on it and you put a foot on the pedal at it's lowest point, with your foot parallel to the ground, your leg is fully stretched.

This does mean that to put a foot on the ground you have to keep your other foot on the pedal and slide forward off the saddle. The posture of your leg for pedalling will be way better getting more power to the bike and reducing stress on the knees.

Make sure your foot rests on the pedal with the front part, like you can see bike racers do. That allows for more muscles to work on pushing than when the pedal is under the middle of the foot.

Beware that raising the saddle does change the front-back balance a bit as your weight sits higher.

I read you bike down stairs, if it is only one or two steps that is not going to change much but if it's a full flight of stairs that might be a problem.

Frankly I'm 52 years old and have been riding bikes since I was 3 and I have not ridden off more than a couple of steps at a time for the last four decades, it's just not something I wish to inflict on my bike, nor on myself. Hopping down half a meter while at speed is no problem, but really riding of flights of stairs? You are a braver man than me.