I'm just about to head off to see if I can get a frame. As you say, I've got a full kit of components though there'll be the usual addition of cables and brake blocks and other stuff, just to make it expensive anyway. Funny how the little things add up isn't it. It'll be fun building another bike, having been able to do it in ages.
As for the dangers of going too hard too early, man do I know that one. I seem psychologically tuned to underestimate my progress and push too hard, but that can almost certainly be said of most if not all people. I've dropped out for short periods over the last few years and it's been mainly coming back and pushing too hard that's nailed the coffin lid back down each time, so I'm used to it and have been pleased this year to see me taking it quietly if that's what the body wants. Of course, 'quietly' on a fixed gear is a bit of a misnomer sometimes because riding slower means pedaling slower which can mean higher pressures on the pedals (depending on the circumstances) which means it's not actually taking it quietly at all. One of the things I love about fixed gear is the inability to hide... but it's also one of the drawbacks. This is where having a good geared bike for those 'lazy' days or extreme days or recovery days is a plus (besides, I enjoy both sides of cycling).
I've got a pretty good idea of what I 'should' be doing and how to do it, my son is a physical fitness professional so I can draw on his knowledge (and that's actually made a big difference at times), my big problem is that I'm just a typically daft human being who only does the correct thing accidentally or under protest.
Diet, that's another issue. Now that I'm living on my own (plus two cats but they don't count), cooking has become a chore. I'm working on changing that (wot, take-away food ain't good for you? Who'd have thunk it?) but unfortunately, most recipes you come across require too much work, too many ingredients and too many techniques. A bit of meat and a salad is good, but not every night. Still, I learned to fry salmon properly the other night which is a bonus.