Thank you for the quick reply, Frederic. That's what I was afraid of - the only "reasonable" way to get this info from ST3 to MTB is in text-only fashion (added to MTB notes) and losing the value of the statistics. I didn't realize that the <distance totalmeters=""> and <duration totalseconds=""> items from ST3 were used anywhere in MTB - I looked, but could not find. So, I'm not sure I understand what you mean when you said "I think creating those additional fields would require a LOT of work as...
Add 2 manual fields + support for them w ST3/fitlog import