I have been considering converting a late 1980's, early 90's Peugeot 12 speed, to a fixed gear bike. I am new to the fixed gear phenomenon so my question may seem naiive. The question is, why wouldn't I just leave the current configuration, keep it in a high gear, and resolve not to change gears or coast? Does it have something to do with the chainline?, efficiency?

I should point out that I am 59 years old, in reasonably good shape, but at this point should I even be considering a fixed gear. I am intrigued by the potential fitness aspects.

I mostly ride my Sirrus Comp at present, sometimes the Peugeot, and some times a Trek Navigator 500.