The first thing I'd say is, 700 hands is way too small a sample to know whether you have leaks or not, but 39bb/100 is certainly not your real winrate - if you're really winning 20% of that, you're doing very well.

I'd also be wary of playing 50NL right away - the poker landscape now vs. 2006 is irrecognisable, and even though you used to play 200NL you're probably going to find that todays 50NL players are considerably better than you're used to, not to mention you haven't played in 7 years.

Good luck with the grind - post some hands.