Just my 2 cents (I'm no "expert")
1) The strange feeling is probably your dive response starting up, restricting blood from lungs/heart going to the legs and arms - That's when "freediving really starts" so to speak... That's why you get lactic.. because the muscles work with insufficiant amounts of oxygen.. On average I think most people would only be half of the way to a BO, but that is very dependent on the specific person. Without warmup, in a pool, I can sometimes get that feeling 20m into i dive, but can go on much further - if I don't bail out because of CO2/discomfort. You can read abot the mammalian dive reflex (wich should probably be called "respons" instead) here:
Mammalian diving reflex - Wikipedia, the free encyclopedia
In the pool, you can continue after vasoconstriction, but you NEED a buddy to spot you continously... You can not feel when it's time for a BO. What you feel is CO2 buildup and the diverespons... (You know about CO2 right? Otherwise read about BO here
2) From your description right there, my guess is that you probably bail out from CO2 buildup discomfort... . Wich is approximately the same for you on full and empty lung (perhaps a bit higher on emptylung - if you can get used to the emty-lung feeling, though...) However I can not explain why you don't get lactic, since your dive respons and vasoconstriction "should" be stronger and kick in faster on emptylung... Perhaps some of the emptylung divers can explain.
Do you hyperventilate before the emptylung walk, because that might delay the dive respons significantly...
How far do you walk, meters/minutes?
Doing emptylung apneawalks can be hard work, be carefull you don't burn out psychologically... I once did get a little burned out, and changed for a more relaxing-orientated training scheme for a while wich helped (CO2/O2 tables in bed, focus on relxation)... Just a good advice: Be process-orientated, not too goal-orientated...