Welcome

The itch to replace it with a fixed gear is getting really bad though, I don't know if it's the look of them or just the idea of it. My question is, how realistic is it to cycle almost 40km a day on one? I'm pretty fit but I don't want to destroy the bike or my body in a few months.

It takes a little getting used to and it's a bit of a grind from the off, but you'll get used to it soon enough and will love it from day 2. I didn't commute by bike at all, but then got myself a plain old fixie and do 18 miles each way each day. If you're fit enough you'll be fine. Have fun :)