After a few failed opportunities I've managed to do a comparison of pre- and post-run HRV using Lepo as outlined a few posts ago. Of course snapshots aren't enough but from yesterday to today there's been a slight increase in the RMSSD value. This suggests that I'm not systemically stressed after my slow 18km. That's good as a long slow run shouldn't do much harm. As I gather more data hopefully some norms and trends will appear.