The more I dig into this stuff, the more Orgel's "Evolution is cleverer than you are" rings true. I just ran across that in Nick Lane's 'Transformer' (highly recommended, BTW) and realize I'd gotten it wrong, thinking it was 'smarter' not 'cleverer'.
For anyone else, it seems pretty clear that whatever's going on, it's not just lactate. Exercise also raises levels of BDNF (brain derived neurotropic factor) and clotho (pretty sure it's with a 'C'; the allele is 'Klotho' with a 'K') which is one of those tantalizing things the longevity people have been going on about. And confusingly (to me, at least) BDNF doesn't seem to be CNS-only.
Have I mentioned I find this stuff fascinating? Trying to pick up the organic chem I need to understand reactions like glycolysis and allosteric changes in enzymes - and haemoglobin - which gave me a serious case of the WTFs.