I'm surprised you seem to think you're getting slower, not faster with more time. I would think you would start getting more fit, although perhaps getting bored with the ride, especially as winter and dark days set in. Are you using battery lights or generator lights?

I second, or third the idea of buying a "road bike"... Low 20 lb range, with the gearing you need. You do need to be prepared for flats, and try to avoid obstructions in the road, although there is a group of road-ish bikes with a bit heavier duty tires which might be considered.

You could also get a commute/light touring bike like Surly, but it isn't needed.

I've used toeclips for years and years. Excellent for commuting, and gives you a little extra edge. This year, I finally converted to SPD clipless bike pedals (also allowing some walking, but perhaps not all day shoes). I think they made a significant difference in my biking, and I have put them on everything I ride.